Totally depends on the ride.

Solo- I dont stop until I need water. 20-30mi- varies with weather/
Group of adults- I stop when others want to. I am almost always the most comfortable riding the farthest in the groups I ride with, so I roll with the schedule of others. Its usually 8-10mi between bar stops.
Group of teens- When I ride with the mentor group of teens, we stop at preselected locations. They are 12-19mi apart, depending on route.
My 8 and 12yo kids- when they want. We ride to find things(creeks, ice cream, playgrounds, etc) so we typically ride until we find what we planned to find.