I've been riding about 3 days a week. Supposedly if one gets one"s heart rate up for about 20 min 3 or 4 days a week that will get one into shape....
Anyway...I do a 10 min warm-up and cool down. In between I ride hard 1 min, and light 2 min for about 20 minutes.
Also, I ride once a week to a place ~3 miles away and then ride back a few hours later. I don't try to ride hard, because I don't want to get there sweaty...but it is a little bit of time on the bike....
I don't have a huge amount of available time...preferably I don't want the workout to last more than 45min to 1hr.