I've read that women spent two or more hours a day grinding grain for their family in the medieval period, probably the most time-intensive labor they engaged in.
This strikes me as odd for the simple reason that they didn't have to. I've eaten cooked whole oat, buckwheat, wheat, and barley groats before. None of them need to be ground to be consumed, though you need to cook them somewhat longer compared to using ground grain when making porridge or a soup.
So this tells me they were doing this because they (or their husbands) valued bread or porridge made from ground grain much more than they valued those 2 hours of labor.
But how can that be? It must have taken a good amount of labor to stay ahead of the growing/gathering/cleaning/making that needed to happen to keep life going in a medieval village. Could they really afford to throw away two hours a day grinding grain?
I'd be interested to known where you read that. The only reason I can think that somebody would mill their own flour rather than go to a mill would be if they were in a particularly isolated community without physical access to one, or if they were trying to avoid what they considered to be a particularly unjust multure. Multure was the fee charged by a miller or mill owner for the use of the mill, and was typically levied in a proportion of the flour milled. This was at a rate set individually by each miller and so there's not one 'set' multure. At Wareham, for example, the multure at the town's watermill was 10%, while the mills at the city of Chester charged 15%.
I talked a bit about multures here, and that talks about one of your options if you thought the multure was too high: simply don't pay it! That answer references a petition to the king from the burghers of Chester, asking for his support to enforce payments owed to the city's various mill-owners by farmers from the surrounding villages who were underpaying, or simply not paying, their multure. What this petition does tell us though is that members of a farming household were most often perfectly willing to travel to the nearest town or village with a mill in order to mill their grain.
The most useful occupation of an English peasant woman's day would likely have been spinning and/or weaving wool, the cottage industry which formed the lynchpin of much of the Medieval English economy.