I always carry two full 24 oz bottles. On a 35 mile ride in hot weather I'll run out and stop for more and in cold weather I may not finish one. I let thirst be my guide. It is, after all, the sense that has evolved over millions of years to guide us as to when we need to drink. In warm weather the rate of water loss is high so you do need to mindful of it and drink at the first indication.