Longest ride in one sitting was 8 hours, only stopped for gas.
I have gotten quite used to my zx6r and my body has conditioned itself for the aggressive seating position. It is usually the bike that needs to stop before I do (fuel etc)...
When I first started riding I couldn't go for very long, bad posture and bad technique. My wrists, back, knees, and even hips would be stiff and sore after a short time. But after my body conditioned itself to it and I learned proper posture and technique I can ride for as long as the bike will go without issue.
I guess my ass still gets kind of sore though.