Because history loves you and wants you to be happy, your question is actually two questions. Because history also has a sense of humor, one of those questions was also asked by Adam Smith. (That Adam Smith.)

So we're going to look at: (1) What did a grocer do? (2) Why did training take seven years?

Concerning grocers

Today's grocery stores sell what we think of as groceries, yes--food--but often, also plenty of non-food products, like greeting cards, insect poison, and Cheese Whiz.. Guild and court records, especially from the late Middle Ages and "earliest modern era" (heh) suggest exactly the same thing.

One major difference from today is that a lot of those additional responsibilities were technically considered other professions, most commonly apothecary and/or pepperer. That was very common for crafts at this time--medical practitioners in particular were known to sue each other for usurping their professional territory.

Apothecary and pepperer were also overlapping professions; pepperers dealing with spices and apothecaries sort of overlapping with our idea of pharmacists...which meant selling a lot of spices and mixtures of spices.

So you can see how grocery merchants would easily delve into those crafts, too. And indeed, in London, apothecaries and pepperers were subsets within the grocers' guild for centuries. (The apothecaries gained independence, so to speak, in 1617.) In 1393, the guild even sought legal permission to forbid everyone [else] in the city from selling spices unless they had been examined and approved by pepperers or garblers, specialists within their own guild.

And grocers' overlap with other crafts didn't stop with edible things. In the late Middle Ages, product lines could include sponges, pots and pans, and apparently lead in bulk quantities.

Selling more products obviously doesn't explain the need for more training. That falls into a few categories, of which the most relevant are probably quality control and math.

City-level merchants in the Middle Ages were typically responsible for quality control of the products they sold directly to customers. We walk home from the grocery store with a bag of grapes and find out one of them in the middle is moldly; we shrug and either throw them out or cut the stems very, very carefully. Medieval legal records show merchants subject to public humiliation punishments for such a mistake (or "mistake").

And of course, there was also the personal need to sell quality products. Since grocers were almost exclusively merchants, that meant not being scammed on the products they bought as well.

Both the legal and economic aspects contributed to the skillset required of grocers. They had to be able to study their products to make sure what they were selling and what they were buying were what they thought the merchandise was. That meant having to understand what good product looked like, how to identify and avoid common scams, and the arithmetic needed to calculate bulk versus individual prices. In some cases, too, currency conversion rates.

The Zibaldone da Canal (zibaldone is the way cool literary genre somewhat comparable to a scrapbook or your "Random Stuff" computer folder) is a really neat source to see some of the things those skillsets included (and is available in English translation, although I don't think online). It has more to do with merchants dealing in foreign goods, but the math and antifraud lessons are pretty impressive.

But...seven years? Yes, seven years. Because:

Concerning apprenticeship

The cheapskate answer is, “Because seven years was customary for apprenticeships across crafts, and in some cases, it was a legal requirement.” Of course, on one hand, enforcement could be…flexible. On the other, there is still the more interesting matter of why seven years was the standard.

Naturally, this question is even more interesting because of the historiographical debates surrounding it. I'm going to come down pretty hard on one side of it even though I'm not an economic historian, because the arguments and evidence for the different sides run parallel to the debate over the medical trades that I do know quite well.

First, Adam Smith and some modern scholars have asked "why seven years" in the context of passing value judgments on the guild system--whether or not it stunted economic growth, technological innovation, etc. Other historians follow the methodology of "It certainly made sense to people for hundreds of years, whether or not it was 'good' from a modern economic standpoint; for what reasons did it make sense?"

As implied above, apprenticeship was a legal status. (City-wise, or guild-wise.) Job training was only one part of it. One classic argument is that cities/guilds wanted to limit the number of practicing craftsmen (occasionally women) in order to avoid an oversupply.

With subsequent research, this theory falls apart. For one thing, there were still far more "graduated" grocers or barbers or whatnot than could have their own shops (master-ships were often numerically limited, and inherited). Plenty of people, most people, spent their career as journeymen or other subordinate workers in someone else's shop. Another problem with this theory contributed to the first: particularly well-off masters might set up a particularly skilled apprentice with their own shop--a franchise business of sorts!

A second traditional argument for a seven-year apprentice stint suggests that apprenticeship consisted of two phases. First, the learning phase. Second, once the apprentice had learned enough to practice the trade on his own, free labor for the master (as payment for the training, or because masters had a lot of power in the guilds and set the rules; take your pick).

The flaw in this idea, which seems pretty reasonable at first blush, is the likelihood of the time/money investment not paying off. Urban former-apprentices absolutely did head to other towns, to villages, or even back to their home village to practice the craft. And while the ~50% child mortality statistic is calculated only to age 16, apprentices could certainly die or become disabled and unable to work during their training.

So if those two models of how apprenticeship worked aren't viable, it's time to flip the question. Instead of emphasizing the "why" of a static thing, we should look at:

What do you mean by "training"?

In the US, a typical master's degree in history requires 12 classes, or the equivalent, in your discipline (plus whatever language etc courses you need). If you're a full time student, that more or less means two years. If you're a part time student, that can mean three, four, or even more years.

Both intensities of students build the same skillset--get the same training. But one group packs the education into a shorter time frame, generally because they don't have as extensive work and/or family obligations beyond school.

So, what is a (US) master's degree? Two years of training? Or four?

A similar principle applied to apprenticeships. What we think of as "education" was spread out longer, because other things were being done at the same time. In this case, free or very, very cheap labor for the master. You don't need all the skills of a master carpenter to do basic carpentry, after all. (Ah, high school shop class...)

However, I don't think the custom of seven years for apprentice training was entirely based in economics. Guilds themselves served far more purposes than craft regulation in medieval/early modern cities. Including in city law codes!

Most scholarly attention to non-economic aspects of guilds has related to religious responsibilities, especially the requirement to stage a particular day's public religious drama. ("Oh, it's Corpus Christi this Sunday? Time to pull together a few wagons to make a stage!")

They were also formal and informal social clubs of a sort. On the formal level, they hosted activities limited to (and often with required attendance by) guild members and possibly families. On the informal level, it wasn't unheard of for the clientele of a particular tavern to be primarily members of a single trade.

It's not like we have fifteenth- or sixteenth-century diaries where apprentices spill their hearts about their social lives over a decade. But it seems reasonable that the apprenticeship as a laborer was also an apprenticeship as a guild member--maturation, socialization, learning what it meant to belong to the group socially as well as economically.

I know, I know, the theory of apprenticeship isn't as sexy or exciting as learning how to sell counterfeit spices without being detected detect any counterfeit spices before you buy them. But if you want to know why grocers needed a seven-year apprenticeship, it's as important to look at the idea of training as to consider what activities were involved.

(Apologies for the delayed response!)