My normal weekly schedule is:

M-off
T- 75
W-50, race pace.
T-off
F-30, moderately fast
S-100+
S-25, recovery

I have a "Saturday Century" group of friends that varies between 2 and 15 riders. I occaisionally do a double on Saturdays or add another century ride on Sunday. If I am away at work, I find a gym with a spin bike and put in the approximate ride time in.

I think I could do less and still be in "Century anytime" condition.