I've been testing this and have had consistent results:
In the week, I play pretty much the same time after work. Whenever my grapes are mature I harvest and juice them without fail. I have a 30 tile grape patch and I manage to put 2 x harvests worth into a barrel from empty. Every time when it's the 3rd harvest's turn, the juice has turned to wine already (36 litres).
As far as understood it, whenever you add more juice to a barrel, the maturity counter zero's. I am not sure of the extent that crashes may have influenced the delay I may have to harvest asap after maturity due to the fixed time I log in, but can not believe it to be that big and even if it was, then I would argue the maturity rate too fast.
If this is not the case, then would it be a reasonable request to ask that the maturity counter be zero'd every time juice is added to juice in a barrel?
I mentioned wine to vinegar in the subject as, even though not tested, I would suspect that the same settings might be in play.