It's not the miles, it's how often you stop. I've got about 1600 miles on my brake pads, also disc, and I think they're close to being ready for a new set. I figure I would have gone through five or six sets of rim pads in that time, so it's all relative.

Re: front vs. rear: see http://www.sheldonbrown.com/brakturn.html . The only time you should use the rear brake is when the front isn't working.