I don't think there's a universal answer. My commute is about 7 miles, but there's a big frigging hill in the middle of it - so I don't ride it as much as I should or would like to. Assuming it's flat, and there isn't extreme weather - I would say 8-10. That goes down for me with extreme weather and/or hills.

How likely is your job to move again if you purchase a house?