I always stop every 1.5 - 2 hours or so, but usually limit my stops to just enough time to use the restroom and/or fill up my water bottle. Most of my stops are under 5 minutes. I also listen to my body, though, and have been known to stop for 20+ minutes to cool down, let my stomach process something too rich or sweet that I ate, etc. There are no hard rules for me. It depends on how I feel. I have to admit that I think that it is NUTS to stay in the saddle for 5+ hours without getting off the bike at all, though. Nuts, I tell you.