Haven't worn my first set of MT21s out yet so dunno what I'll get.
Extrapolating from what I see now it'll probably be about 4000 miles.
I also tend to replace a knobbie way before you have to since it makes no sense to have one with 1/4 in knobs. I have ridden the street in the rain this winter ridden mud (the clay sticky stuff we have here), rocks, grass and hardpack. They tend to do OK on just about everything.
On the whole, I'd rather ride a knobbie a bit slower on the street than keep getting spit off riding a street tire in the dirt. The MT21s have surprising tractioin on the street, they have good feedback if you push them to hard in a corner (squirmy feeling), haven't done anything dangerous and don't howl on the street. They are certainly noisier than a street tire but nothing bothersome.
My DRZ gets almost exactly 50/50 street/dirt use if that's any help.