I think it depends on the scenery, the company, and how many hours of daylight there are. Nice scenery and nice company makes it more likely to take breaks and have fun off the bike. Longer days give you more time to accomplish your mileage.

If you are by yourself and don't have any interest in what's around you and don't mind doing the same activity all day long, and the days are 14 hours long. 100+ miles a day is simply a matter of mental fortitude and having a butt made of leather.