This is a fascinating, but complicated question. It forces us to narrow down the inquiry to specific times and places, because it's impossible to generalize the entire medieval period - certainly a person might have less work at the height of the black plague but that might not mean leisure time, I'm sure you'll agree! - and the same must be said of the early modern period - however you may define it - and even post-industrialization, there were large variations in times and place.

On the whole, it does seem that free time or leisure for an average worker has been reduced, generally, from the medieval period to the modern day, but not in a strictly linear progression. There have been estimates that present the average hours of work done per year per average male worker in particular centuries. There are variations throughout the centuries, for instance the average hours put in in the 13th century are higher than those in the 14th, but the estimate given, assuming at least a 9.5 hour work day working 2/3s of the year across the whole medieval period in the UK is given as 2309 hours per year. From 1400-1600, the average is given as 1980 hours.

There are of course exceptions to this model: James E. Thorold Rogers in Six Centuries of Work and Wages argues that, at least in the 13th century, artisans worked a day likely no longer than 8 hours, with enough spare time that there's evidence that these artisans supplemented their income and subsistence with wage agricultural work, and even sold some of their produce to their artisan bosses. Later, in the 17th century, he puts the work day at between 12-14 hours. So again, this is likely not a simple linear progression.

However, the cumulative change is still striking: the average yearly hourly load number nearly doubles by 1840, to between 3100-3600 hours, based on a 70 hour week. The average for the United States is comparable.

Why the enormous change?

You've alluded to it in your OP: industrialization. Agriculture, for centuries, had been the primary production economy in western Europe, but but the 1820s and roaring on until the end of the century, American and British manufacturers would take a larger and larger slice of the pie. This has been exhaustively studied, not simply because the hours of the workweek rose, but because the shift from an agricultural to a maufacturing economy changed nearly everything about your average worker's life. It changed the nature of their education, the food they ate, how they ate it, it changed the idea of the home, the idea of the family, the idea of holidays, the idea of their power in an economic relationship, it changed their politics, it changed their militia requirements, it changed ideas about sex and sexuality, about worship and entertainment and I am not exaggerating when I say it changed everything. One of the classic works that discusses this enormous multifacteted process is Charles Sellers' The Market Revolution. Sellers is not perfect and in some respects the work is dated, and it's cheerfully devoted to specifically American changes of the Industrial Revolution, but it's a good start. He sums up a vast amount of conflict thusly:

Profound cultural differences arose from these contrasting modes of production. The market fostered individualism and competitive pursuit of wealth by open-ended production of commodity values that could be accumulated as money. But rural production of use values stopped once bodies were sheltered and clothed and bellies provided for. Surplus produce had no abstract or money value, and wealth could not be accumulated. Therefore the subsistence culture fostered family obligation, communal cooperation, and reproduction over generations of a modest comfort.

Part of what Sellers is loosely describing here is what economists call a backward-bending supply curve of labor; basically, when wages rise or needs are met, workers stop working. The thought of doing that today, of going to our boss and saying "hey, I've done a lot of good work today, I'm going to head out" at noon, even if we have done a day's worth of work, is profoundly alien to most of us. But this is a process that has been noted, and it was complained about by early capitalists and industrialists, and even has historical precedents in guild systems and others. Essentially, one of the major changes brought about by the Industrial Revolution was instituting a system where workers had no power to withhold their labor. It was a long, messy process the specifics of which are beyond the scope of the question, but while I may have implied above that this was done by greedy mustachioed factory barons plotting alongside bought-and-paid-for politicians, the truth was far more often that converging aspects of the new economy presented opportunities that capitalists (the definition of capitalist I'm using here is the open and intentional union of the ownership class with the political class, not simply the rich - capitalism is the union of political power with property power) were more than happy to take advantage of.

So what were these old freedoms that workers had? For one thing, holidays. But the erosion of the church-calendar year was already ongoing by the time floor bosses clocked their workers at the steel mill, that was a process jumpstarted by the Reformation and the chipping away of the dominance of the Catholic church. The medieval calendar year had, according to Francis and Joseph Gies, thirty or so holidays in the year. Some of those were multi-day, multi-stage holidays that allowed people to celebrate for days in a row. The conception of time implied by many medieval writings was one that counted time between holidays, or dated events to their proximity to a holiday. In the 16th century work of the "poor knight" Gotz von Berlichingen, for instance, he gives the date of the day that he lost his hand to a cannoball "the day after the feast of St. Jacob," and remembers that it was a sunday, but he doesn't give a calendar date.

Some of these holidays also had deliberate and tolerated subversions of rules, both of polite society and of political power. "Topsy-turvy" or "world turned upside down" themes reigned in holidays like All Fool's Day, and allowed poorer folks to prank or make jokes of their betters. But even a more casual relationship often ruled the notion of work. A 16th century curmudgeon, the Bishop of Durham James Pilkington, wrote of the work ethic he observed:

The labouring man will take his rest long in the morning; a good piece of the day is spent afore he come at his work; then he must have his breakfast, though he have not earned it at his accustomed hour, or else there is grudging and murmuring; when the clock smiteth, he will cast down his burden in the midway, and whatsoever he is in hand with, he will leave it as it is, though many times it is marred afore he come again; he may not lose his meat, what danger soever the work is in. At noon he must have his sleeping time, then his bever in the afternoon, which spendeth a great part of the day; and when his hour cometh at night, at the first stroke of the clock he casteth down his tools, leaveth his work, in what need or case soever the work standeth.

Guild journeymen of the Holy Roman Empire in the same century had a tradition they called Guter Montag or Good Monday: their masters would allow, and sometimes even pay for, a bout of monday-afternoon drinking. Every monday. Post-industrial writers cite this as evidence of the poor work ethic and of the inherent laziness of the worker before the industrial economy forced them into stricter discipline, which is an idea of course that has a lot of problematic aspects to our relationship with work today.

We see in various parts of the economy methods by which workers challenged unfair or overbearing bosses. Wage laborers often simply left the job if they felt that they were not respected. Michael Behaim, an apprentice clerk in the 1520s, wrote to his guardian of his master taking advantage of him, and using him as a mere shop-sweeper, instead of properly teaching him the trade. His guardian eventually found him better employment elsewhere. Petty suits between apprentices or journeyman and their masters, and vice versa, were commonplace. Some wage laborers expected not only a day's pay, but midday breaks and a meal, and we have records of complaints when the meal is tawdry or the break is short, or even that their supplied tools are in poor condition, etc. Peasants were expected, sometimes, to work a number of days on their lord's property, and the demand for their proper recompense, in kind or pay, was loud and often belligerent.

Juliet Schor even argues that within the past two or three decades, the relationship between the worker and the workplace, the expectation of work and productivity, has changed even more, and that Americans are working more with less leisure than similar jobs had in the 60s or before.

To give a long, long story a short summation: yeah, it seems like modern folks work longer for less leisure than at many times and places in the past, but the more compelling observation, in my opinion, is the erosion of the ability to wield power in the workplace. Wage conditions and contract work in boilerplate leave little room for what a 16th century journeyman or even a 14th century farmer might expect as their just treatment, and the tools for hitting back today are curtailed, legally and culturally. But we should bear in mind that this is not universal, and work-conditions even just limited to a few centuries in Europe fluctuate widely.

Sources below.

To add to what u/PartyMoses has said, the debate among historians of leisure is very similar to parallel debates within cultural and social history about many other concepts or terms: namely, before we had a concept like "leisure", was there something empirically real in the lives of human beings that we could call "leisure"?

There's a well-known debate in the field of leisure history between Peter Burke (no relation) and JL Marfany that illuminates this question--Burke's original essay "The Invention of Leisure in Early Modern Europe" in the journal Past and Present in 1995 and Marfany's rejoinder in the same journal in 1997 (followed by an additional response to the response by Burke).

Burke argues that while late medieval serfs may have not been constantly engaged in labor, they did not conceptualize the time they were not working in terms of what we think of as leisure. Leisure, Burke argues, is specifically defined as time off from work within the regimentation of time that characterized industrial societies between the late 18th and late 20th Century. All the things that we associate with leisure--hobbies, private time with family and friends in domestic settings, organized social occasions like drinking in pubs or watching sports, the consumption of culture (reading, watching TV, going to the opera, etc.)--in Burke's view only really exist as a distinct form of sociocultural experience in modernity and are only defined as the opposite or alternative to work in industrial regimes.

Burke observes that late medieval European peasants did generally engage in distinctive kinds of cultural or social activities during "not-work" and didn't divide or imagine their days in those terms. The only thing, in his view, that seems somewhat like leisure are festivals or fairs and these, he notes, were mostly associated with holy days on the Christian calendar and with religious observation--that they might have contests or spectacles built into them was a sort of sideline. Burke also points out that the origins of leisure were very distinctly associated at first with the aristocracy or "gentlemanly" classes, who in the early modern period increasingly found themselves with less and less to do and yet in some cases with more and more resources available to do as they pleased--and that this is the era in which many new forms of "leisure-like" activities started to emerge. Industrialization allowed the concept to spread to new middle-classes and eventually workers--but leisure was also something they had to fight for.

Marfany replies: come on, late medieval peasants drank with friends, they had boisterous conversations with their families and neighbors, they took breaks from agricultural work and just lazed about, they went swimming or daydreamed--and all of this was "leisure" even if it wasn't conceptualized as such by them and it wasn't as tightly organized or as various in its forms as in the modern period. In essence, Marfany says, "the concept might be new, but it is giving a name to something much more universal".

Since the 1990s, you can find echoes of this debate all up and down the historiography of leisure. Were Roman gladiatorial matches "leisure" of the same kind as our modern football matches? It looks like it at first--but some historians who study gladiatorial matches argue that they were more akin to religious ceremonies or had other purposes in the minds of the participants and spectators than what we would think of as leisure. And yet other historians reply as Marfany does: if it looks like a duck and quacks like a duck, it's a duck: people seem to have gone to the matches for fun, they certainly weren't at work, there's nothing that precludes a fun, leisurely activity from having additional political, spiritual or cultural meanings.

You might think the way out of this is simply to ask, as the OP does, how much time people spent working and then figure that the remainder is all leisure. The less time working, the more leisure, certainly? And yet I think we today recognize first that in our current economy (all the more so in covid-19), distinguishing work from not-work can be remarkably difficult. Am I working in answering this? I am an employed historian who believes professors have some form of public responsibility. But I'm not at all required to do it--no one in my workplace knows that I do. I can disappear for months at a time from r/AskHistorians and no one sends an email to my employer. So I must be doing it for fun as a kind of not-work. Which is also true but...not really the same kind of not-work as when I'm out on a walk or working on my photography.

It isn't necessarily easy to separate work and not-work in pre-industrial societies either. In the region of southern Africa that I know best, there's a harvest ritual that involves threshing grain like sorghum or millet--it's 'fun' in the sense that people get to recite amusingly insulting songs and poems about each other, there's some drinking involved--but it's also labor. Marshall Sahlins has famously suggested in his essay on "Stone Age economics" that pre-Neolithic hunter-gatherers had the most leisure time and the least work time of any human society, and he certainly has a point--agriculture makes extreme demands on human labor, especially seasonally. But again take a confusing bit of evidence, since we don't have much direct observational knowledge of their lives--were cave paintings "fun" or were they "work"--a practical exercise intended to give spiritual aid and encouragement to hunters?

I think it's reasonable to say that modern life between about 1800 and 1990 was the most regimented in terms of time in human history, and the boundaries between work and not-work were the most formally defined and enforced. And as such, they were also something that employers and rulers could restrict or constrain, and in some ways, human beings both had more to do when they were not working and yet less time to do it in. We may be in an era when the boundaries of work and not-work are breaking down again, but that is seemingly not returning time back to people--thought that is a debate outside the bounds of r/AsHistorians.