I've commuted by bike from 72 miles round trip to just a few miles. When I finally settled down to buying a place to live I deliberately bought one near a bike path that would give me a slightly more than 20 mile round trip or if I'm in a hurry a streets route of 16 miles round trip. It really does come down to an issue of time, as many have mentioned, and for me that is roughly 30- 45 mins each way, which feels just right. Commutes of an hour or more each way are fine for a season or a year or so but after several years start to feel like an obsession rather than just a way to get to work.