This is one of those history questions we naturally find ourselves asking, yet it is so difficult to answer with the source base we have. Now it kind of goes without saying, but I'll say it here anyway, that we have barely anything by way of subjective written sources (i.e. letters, diaries, memoirs) emanating from the peasantry themselves - though quite a fair number of people from peasant backgrounds received the necessary literate education to write such things (manorial court documents from 13th and 14th century England show very clearly that a significant portion of serfs were sending at least one child from their family to monastic and cathedral schools), they tended not to remain peasants once they'd finished their schooling i.e. the ultimate social mobility story in this case is that of William of Wykeham, the son of a free peasant father and a serf mother, who after his schooling (paid for by his lord, the bishop of Winchester) became a clerk in the service of the bishop of Winchester, an architect, a government minister and eventually bishop of Winchester himself. A lot of the sources we have on peasant life come in the form legal records (manorial court rolls, land charters, some royal legislation etc), which detail things the status of peasants (whether they were free or serfs), what property rights, obligations (rents and labour services they owed) and personal freedoms they had, how the land was managed, how their disputes were resolved and the like. We also have a fair amount of archaeological evidence (the excavations done at the villages of Warrham Percy in England and Cosmeston in Wales have been particularly illuminating) which can tell us quite a bit about the material living standards of peasants i.e. how big spacious were their houses and how many rooms/ floors did they have, what kind of appurtenances they had (pottery tends to survive best), what kind of diets they had (we can tell how much meat and dairy they got from skeletal remains) etc. We can also glean a few insights into peasant life from passing references in narrative sources like saints' lives or works of vernacular literature. But generally all the other sectors of medieval society are better documented, and for something as highly subjective as quality of life the minds of peasants are much more impenetrable for historians than those of, say, nobles, clergy or townspeople.

It is also worth remembering that this is a subject that has often been approached in a very ideological and rather black and white manner. Liberals and Marxists have tended to be wedded to the image of wretched and oppressed peasants eking out a bare bones subsistence living while their lords creamed off all the surpluses and denied them some of the most basic freedoms. Romantic conservatives (mostly, though far from exclusively, Catholic or High Church Anglican ones) and heterodox socialists (most famously John Ruskin and William Morris in the Victorian era and to a lesser extent Richard Tawney and Karl Polanyi in the early 20th century), on the other hand, have been attracted to the idea of a lost communitarian bucolic idyll in which before the watershed provided by the agricultural and industrial revolutions of the 18th and early 19th centuries the peasantry were prosperous, self-sufficient and independent, had a strong sense of community and mutual support and enjoyed much greater leisure and a more pleasant living environment than their working class descendants living in towns and cities. Professional historians in recent generations have generally tried to be even handed, though quite often they'll inevitably lean a little bit towards one or the other.

All of those caveats out the way, I'll provide a brief sketch of what it might have been like, particularly focusing on how it might have varied and how it might have changed over time.

In the early Middle Ages (500 - 1000 let's say), material standards of living for peasants, certainly in terms of quality of housing and access to consumer goods, definitely do seem to have declined quite drastically following the Fall of the Western Roman Empire (see Bryan Ward Perkins' brilliant book "the Fall of Rome and the end of civilisation"). However, since population seems to have been declining (up until the 7th century) or remaining fairly constant (7th to 10th centuries), which relieved a lot of the demand and pressure for arable farming, many peasants were able to have a lot of meat, fish, fresh fruit and dairy in their diet - skeletal evidence from the 10th century North Western Europe seems to suggest that men and women of all social statuses were on average as tall as the average man or woman in Europe and North America today. On the estates of kings, bishops and monasteries, which are by far the best documented (the Carolingian polyptychs of the 9th century, some of which are in Georges Duby's "Rural economy and country life in the Medieval West", are demonstrative of this) there was definitely a complex pecking order of status between free and unfree peasants, but there certainly were a fair amount of allodial peasants, mainly in heavily forested, swampy or mountainous regions, who would have enjoyed a lot of independence but would have had to largely fend for themselves as well given their distance from higher authorities. A lot of peasants, especially the allodial ones living in geographically remote areas, would have been quite lacking in community beyond their families, as nucleated villages and parishes didn't really begin to emerge in much of Western Europe until the 10th century - many early medieval peasants lived in hamlets/ isolated farmsteads and relied on itinerant priests from cathedrals and monasteries.

In the 11th century, a lot of formerly free peasants were reduced to serfdom in what is often called the "feudal revolution" (see the second chapter of R.I Moore's "The first European revolution, c.970 - 1215" for a fairly good introduction to that). which didn't necessarily mean a decline in living standards but certainly brought with it reduced legal status and social stigma. The 12th and 13th centuries were a period of very rapid economic growth in the form of agricultural expansion and commercialisation, and this would have brought mixed fortunes for the peasantry. Those living in already agriculturally developed areas would have experienced new burdens in terms of rents and labour services from their lords (this was especially true in much of Southern and Midland England) who were becoming more and more interested in direct exploitation of their lands to satisfy the demands of growing towns and cities. On the other hand, those moving into areas that had recently been cleared (former forests and marshlands) or colonised (i.e. the Flemish and German peasant colonists in Eastern Europe) could hope for low rents, fairly extensive legal rights and a stable village environment where they could often have their own self-governing institutions and elected officials as lords wanted to attract settlers. With regard to commercialisation, some peasants were able to profit from selling their own produce to towns and then buy lots of consumer goods (peasants in Southern Wales were consuming French pottery and fine wine as archaeological evidence has shown) and consolidate and expand their holdings through land markets. Others became impoverished as a result of excessive subdivision of smallholdings between multiple heirs in areas where partible inheritance prevailed, and were forced to rely on seasonal wage labour performed either for their lords or their richer neighbours (especially common in England). So, in short, for some life their quality of life was getting better in this period and for others it was getting worse.

In the late Middle Ages (the 14th and 15th centuries) it seems that most peasants, those who survived the Great European Famine and the Black Death, came to enjoy a relatively good quality of life. With post-plague depopulation (most stark in England where the population decreased from a probable 6 million in 1300 to around 2 million by 1450, with demographic growth not being really seen again at national level until the 1520s), the average peasant came to enjoy a much more carnivorous diet than they'd had in the 12th and 13th centuries as the demand for arable farming massively lessened. Many peasant houses from this period still survive, which suggest that by this point certainly the more well-off ones were starting to have upper floors, private bedrooms and the like. The effects of the Black Death on land and labour markets meant that the entrepreneurial peasants were able to buy up even more land and establish themselves as small-scale agrarian capitalists (the English yeoman farmers being the quintessential example), while those largely dependent on wage labour were able to negotiate better wages, better employment benefits and reduced working hours. The same factors led to the end of serfdom in Western Europe, and with that the legal stigma around it - though it must be said that in Central and Eastern Europe serfdom actually strengthened in the centuries following the Black Death. Indeed, its been estimated that peasants and labourers in 15th century England enjoyed more leisure time than in any subsequent era up until the 1960s. This was also a period in which peasants had a lot going for them by way of community, with all sorts of communal activities like church ales and festal dances developing as well as new civic associations designed mutual aid, self-help and social activities like parish guilds for both men and women. So from both a material and immaterial standpoint, things were arguably getting quite good the average peasant towards the end of our period.