My commute is only 3.5 miles one way, but on the return trip I do a loop route which increases the overall round trip to ten miles (plus I do a moderately fast paced ride of about 45 minutes during lunch three days a week). So my normal commute regimen is ten miles for the round trip. My guess is I'd still commute every day if my distance increased to as much as 10 miles one way, but past that I'd probably begin riding my bike to the train station, and taking the train in.