I try to go as long as the water bottles will allow. I carry two 24oz bottles with me and during the summer I drink about 1 per hour. In the fall (like now) I drink about 1 every 1.5 hours. When I finish the last bottle I should be within 30 minutes of home or I'll have to get some more water.

3.5 hours is about as long as I ride, and If I'm by myself I don't like to stop. So I guess my answer is 0.