Fifteen hilly miles is pretty a pretty hefty commute done twice a day, every day. I, myself, could probably manage to do it 3-4 times out of the week, but to do it everyday might wear me down quickly. I suppose if you're in relatively good aerobic condition, give yourself plenty of time so that you can cruise if you need to, and have a comfortable enough bike, it'll be fine, and you'll be getting two+ hours of exercise every single day.

My ride is 12 miles one way. I do it 5/5 days a week most weeks, but the ride is easy. Mostly flat. Good back roads and trails. Very little traffic on the outward ride and quite tolerable on the inward one.