I doubt that a whole lot of "average" cyclists post on this forum. On long rides, about every 50 miles is about right for me. My shorter, more social rides are usually 40-70 miles, one coffee stop about in the middle, probably one quick pee stop somewhere else along the way. A good plan is to stop about in the middle of your intended ride, of whatever length, and have a good stretch, and/or do that whenever you start really feeling the chain. It's good to work hard, but also good to feel good while doing it.
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