My commute is 12 miles each way, and it's perfect for me. It also depends on the type of bicycle you have. There is no way I would do that distance on a mountain bike with knobby tires (too slow) or a single speed as I also have hills on my commute. I have to cross several major highways and encounter many lights so my commute time is aout 65 minutes. I'd also be very concerned with the quality of your commute, as narrow roads with no shoulders are no fun and you will tire of it quickly. In my opinion, the key to a comfortable longer commute is to be as efficient as possible in terms of your total weight, gearing, tires, etc. With the wrong setup, even a 4 mile commute can feel long and tiresome.