My ride distance depends on a few variables:
1. amount of time I have
(if it is right after work and time is short, I don't have a problem cutting a ride short to 15 or 20 miles)
2. weather conditions
(we have very high winds here for a good part of the year. It blew 50+mph
yesterday and the forecast is for 60+ today -- I will not ride in those conditions.)
3. how much I have ridden the previous three or four days. (I like to get a ride every other day or so)
4. on weekends I will ride farther than on weekdays
(related to #1 -- weekend mornings around here typically yield lighter traffic and lighter winds;
a 40-mile ride is not uncommon then.)