10 miles.
A bit of a random guess, but it is very much based on how fast you want to ride. gb155 on here has achieved stunning weight loss based on a relatively short commute (roughly this distance IIRC), but at very high intensity (every day is a Time Trial). Not to knock the work he does during the weekends, but he does credit the commute as loosing weight without loosing time. Me, I do a 12 mile there-and-back commute (2 x 6, if you are still confused), again at "every-day-is-a-time-trial" intensity, have upped my calorie intake considerably, and have lost 2" around the waist and 1.5 stone in weight (can't believe I've gone so imperial).
However there is the proviso that you can't eat any more food (or not much more), which can be difficult if the hunger from the riding is too tempting not to address.