I think they should last about 50,000 miles if you don't drive aggressively.
My inner/outer pads wear evenly but I have seen many people complain about the inner pad wearing faster. If this happens I would guess you have a problem with the guide pins being dirty or an issue with the caliper piston not retracting properly.
The rear pads should last twice as long as the fronts, you you defiantly have some sort of dragging brake issue if the rears are wearing faster.
Straight lines are for fast cars...turns are for fast drivers
'04 M3 - track
'06 650i Cab - daily driver
'02 Highlander - kid hauler
'01 330Ci - winter beater