I personally think that a century is if you ride 100 miles on a given ride. meaning you don't come in "home" Example you leave at 6 AM and ride 30 miles..stop somewhere to rest..look at the world etc. get back on your bike ride 50 miles..stop and have a beer or food relax sit around take in the view then get back on the bike and ride20 miles back home. There is no time limit. I personally think if you are "out" cycling and don't stop being on a "bike trip" or sleep for an overnight somewhere in the woods or park then it could take you as long as you want as long as you are still on a "bike trip". But it is still a century. i remember years ago I went on a ride that was 130 miles it took us 15 hours to do. When on the bike we averaged 19 mph. Problem is it was a party ride and we spend 6 hours at a bar playing sand volleyball. To us we still did a century because we rode our bikes to point a to point b without retiring for the night.