I consider a century a 100 miles on a single ride within a 24 hour period regardless of how long it takes. Although I am not sure how long a break has to be before it is considered starting a second ride. Sometimes I will do 95 miles, go home, and then finish the last 5 miles several hours later after a long rest. Feels like I am cheating or doing it on Easy mode, but I still count it as a century.