I prefer rides of at least an hour, but no more than four. Under an hour and I don't feel fulfilled. I can ride longer than four, but the joy starts to drain away after four hours or so, and I'm mentally miserable (although physically fine) by six hours. Most typically, my preference is around two hours.
Translated, my most frequent loop is 27 miles. Halfway back yesterday I was thinking that I have to extend that since I've got it under 90 minutes now. I'm thinking 35 would be about right--another four miles out. Consulting the maps later today.