I've got about 30K miles on the current pads and from the looks of them when I had the calipers/hoses replaced they'll go at least another 30K. The first set of pads was replaced by the previous owners at around 50K.
I had the ALL the calipers replaced on the recommendation of my mechanic (also a friend who I trust) because he said that one of them had a slight leak and appeared to have taken more heat than the others. I also decided that while he was replacing the calipers, it was a good time to replace the flexible rubber hoses, and while he was in there doing that, I felt like it was warranted to flush the entire hydraulic brake system and put in new fluid. Good for peace of mind that everything is new and fresh, bad for the wallet....