I try to drink 1-24oz bottle per hour, but I really just end up drinking about 20-oz./ hour. I've never gotten dehydrated during a ride (that I have noticed - of course ) I step it up naturally when I need to (in other words I don't think about drinking more, I just drink more).

On my hottest ride this year I ran out of fluids (in bottles) during the last 40 miles after the last rest stop (on a 160 mile ride). Even then I ran out in last 9-10 miles. 1/2 hour at the end of a 160 mile ride in very high heat and humidity without fluids ...was interesting, but not really a problem.

For the "desert" comment...FWIW I rode in El Paso for 2 years using about 1-20oz bottle per hour. No problems.