Usually for me, depending on where I get assigned, fuel becomes an issue first. Unless, of course I'm in a target rich location.
But I think I remember reading somewhere (Blair maybe?) that they tried to match the food load to the expected fuel usage. Iirc, somewhere around 2-3 months. But don't quote me on this.
Edit:
http://uboat.net/men/foodstuffs.htm
Per that link, a IXC usually carried a 12 week supply. In game you would be hard pressed, short of being assigned to the sargasso sea, to keep torpedos for that long.