I have done four centuries and several 80-90 mile rides. The times range from a blistering 6 hours with stops to closer to 9 hours (for an 85 miler). The fast one was flat and I was in great shape, only needed 3 rest stops for water. The other was hilly but I was also very unprepared and HAD to take pretty much every rest stop after 40 miles and then spent a long time at the stop wondering if I could get back on the bike.

It all depends on the ride, the weather, the terrain, your physical condition, mental condition and how hard you want to go. Some people enjoy riding slow and soaking in the experience, nothing wrong with that. Some people are pushing to finish the ride quick, maybe set a personal best.