Like Pilgrim said- lots of factors- heavy, roads, riding style, 1 or 2 up, how hard you brake, type of tire, inflation.
My fronts also wear more than the rears, but only by about maybe 20%. So by the time I need a front, I might as well replace the rear, too. I get good tire life. First set of PR4's 18,020 miles, last set of PR4's 16,031 miles (cut short due to a large nail in rear that I had to emergency plug and I wasn't going to push it).
I try to always keep both at exactly 42psi.