I haven't had a computer on a bike for over 5 or 6 years.  Most of the time I go by the duration of the last ride, how I feel, or simply how much time is available.  Right now I'm training for the 24 Hours of Old Pueblo in AZ on Feb 19, I try to get an hour on the rollers before work on the fixed gear (like your own little spin class) and maybe another longer session in the evening depending on the moods of my pregnant wife and 2.5 year old (thanks to both for any time possible).  Mileage seems like an arbitrary measure of my enjoyment and time on the bike.