My commute is about 7.5 miles each way, but I carry clothes and meals for work, and Seattle is quite hilly. I also lose quite some time traversing the entire hospital campus twice on foot to get from the bicycle cage to the bike commuter locker room (shower) and then back to the ER where I work. With that and working 12 hour shifts on my feet I wouldn't want my commute to be much longer. Especially since I am also going to be back in school five days a week and carrying schoolbooks as well.

When I was working "only" 10 hour shifts and not going to school I would've been okay with a little bit longer commute. Sometimes I'm envious of flat landers and I'm definitely envious of those that get to commute out in the country, except on windy days!

tldr version: varies on load, terrain, weather, sleep, type of work you do. Anything over 10 miles each way I think I would feel is "long" unless it was a flat and pleasant commute.