Varies. On open road rides with few or no stop signs or traffic lights, I'll usually stop briefly around 20-40 miles; more often beyond 50 miles -- mostly to stretch my neck and shoulder (old injuries). Pretty much the same with small group open road rides. We tend to stop around every 25 miles. Especially on hot summer day rides. Usually brief. I try to keep my stops to a minute or less on my usual 20-40 mile solo workouts. On longer rides I might stop for up to 15 minutes to eat, but I'm usually standing or walking around a bit to keep the legs alive.
On city rides and some no-drop casual group rides, stops are dictated by stop signs, traffic lights and the slowest riders. Every group I've ridden with eventually splits off into sub-groups or entirely separate groups over this issue. Stronger riders tend to not want to stop because after more than 15-30 seconds the legs start to feel heavy and sluggish. And worrywarts like me will drop out of groups that blow through red lights and don't look every direction before rolling through stop signs. So I tend to regard all city/suburb group rides as casual and expect to either stop more often so we don't drop friends, or let the group go if they want to play racing.