base it on a workable avg spd for You. My very first century I planned on 10 hrs, hoped I could do it in 8, was pleasantly surprised when I did in it 7. Since then I've got my time to 6:15 and was overjoyed, but I Still plan on 8 hours just to be safe.

ps: I've never done an organized century so my rest stops are short and sweet coz it's just me! and organized ride would probably add more overall time.