I generally stop about every 20 miles just to walk or stand around for a couple of minutes, get some water and maybe some nutrition, if it's a long ride. I think walking around periodically helps with comfort on the bike, and I'm all for that, especially on longer rides. When it's really hot, I'll stop as often as needed to rehydrate. I drink out of screw-on water bottles, so it's usually a good idea to stop to take a drink.