Clearly, you'll get no definitive answer here. The group I ride with stops a bit too often for my tastes, but I don't mind the 15-20 minute Coke break at the deli after about 20 miles. If I am alone or with a couple of my buddies, we tend to go farther between breaks (20-25), but there is usually some destination (Starbucks, a deli) in mind. On a longer ride (80+) getting a good lunch is worth the longer stop.
I always carry a camera, so it's not unusual for me to stop for 30 seconds for a shot. But I also carry it around my neck, so there are many pictures taken while moving.