Time is more of a factor than distance. 60 minutes one way might be my limit unless public transit took significantly longer. My commute is rather short at 12 to 15km (7.5 to 9 miles) but it is very urban/suburban with lots of stops, lights, traffic & hills. It's takes me anywhere from 40 minutes (quiet summer Sunday morning to work) up to 75 minutes (winter storm on the way home). Since cycling takes just as long as public transit it doesn't affect my personal or family life any differently. But I commute more for the exercise and health benefits more than any other reason.