I wind up doing 4 or 5 centuries per year: one or two are "organized" (sic) rides, one is usually the graduation ride for the 12-week cycling skills series I coach, and a couple are just either accidental or training rides leading up to one of the aforementioned.

To be honest, I don't really enjoy centuries all that much; I've found that my ideal cycling distance for a single day's ride is ~75-80 miles...less than that and I feel like I missed an opportunity, but more than that and I feel like I'm just a little too desperate in my search for nirvana. But right around 75-80 miles, man, that's perfect!