It depends, too, on the distance. 5 times/week is easy if your commute takes under 30 minutes each way. It's a lot tougher to maintain if it's 1.5 to 2 hours each way. Then there's multimodal commuting. I currently live 22 miles from my work. I usually ride it in the summer, but now that it's winter and there's a train available, I often take the train part of the way in and ride home in the evenings. It still makes for a 30 mile RT commute on the bike when I take the train, but I feel like it's "cheating" to call it a bike commute day when a train was involved.