In my experience, I wore through an XT level cassette in about 1000 miles. Mind you, I'm a 'pusher' and not a 'spinner.' My boss even said so when he checked out my first worn out cassette, told me I push too hard a gear. YMMV.
I've been told by a few people "They take more material off the cogs and they'll wear faster" than a normal cassette if you push too hard a gear. I have no clue how true this is... I will say the Deore level cassette I used did seem to last longer.