Well I go through DH tires that fast becuse they're made out of real soft rubber to be grippy, not long-lasting, so they shread on rocks easily. But your XC tires may well last you as long as you can stand to ride them.
Tires are cheap though if you buy them off the net. Often times $10 a pop. And dont let anyone give you crap about not supporting your bike shop either. Its a business not a charity.