It should be determined by multiple factors say;
How much the player has sold (not grown)
Random AI data stuff
Some kinda pre-planned system at least for years 1-3
I like this idea.
a couple things that could determine the demand could be,
the amount of product the player has sold (if you sell 1000 consecutive bottles of wine demand would go down but if you don't sell it for 100 year demand will hit a peak)
there should be more demand if more npcs like or love said item.
quality ( if you sell 100 low quality grapes demand will plumit as you sell it. Basically demand never goes up. But the higher the quality the longer it stays in demand and the higher in demand it is)
weather ( if it rains for a couple days or every other day for a while, people will want comfort items like tea
And other stuff so demand will go up. But in a dry spell people want flavorful food juicy stuff like tomatoes apples plums and other fruits, wine preserves etc.)
a thing I would want is if you put milk in a keg you get egg nog but it's only in demand in the winter
And if a demand mechanic doesn't get added there could be a special ingredient only found at winter time.
Season ( most seasonal fruit irl is most in demand in the months it doesn't grow in. like apples they grow in fall, at least I hope or that makes me look like a fool. Back to the point apples would be in more demand out of season)
Birthdays ( This would be a small factor, but doesn't it make sense that around villagers birthdays they would buy there favorite foods and items etc)
Well that's all I can think of for now!