Personally, 31 miles is way more than I'd like to do...I wouldn't want to be biking for more than an hour or so before/after work--I value my sleep too much in the morning!

My commute now is about 5 miles one way, and takes anywhere between 15 (at my very fastest) and 20 minutes (depending on traffic/how I hit lights). I figure my upper limit for commuting is about 15 miles one way. That would put me right at about an hour one way, which is manageable.