I've been daily bike commuting for the past three and a half years. I started out with 10 - 12 miles a day, but kept modifying my route to increase the milage. I'm up to riding 25 - 30 miles a day, five days a week, commuting for the past two years. Most weekends I'll get out for at least one 50 mile ride (and am shooting for a 100 mile ride a month), and find that I'm more inclined when I've commited to a group ride or an event.