i have a set of knobbies with about 200 on them and the back tire looks like its wearing out. How long do tires last? I realize that knobbies when run almost exclusively on asphault probably won't last long

I get 2000 to 4000 on tires. If your rear tire is wearing out after 200 I wonder if the wheel is misaligned.

It depends on the tire, and the use.

Softer compound rubber will wear out faster (e.g. knobbies and dirt tires, cheap road tires) especially if used on surfaces which tear them up, like rocks or rough pavement.

Harder compounds will last longer, but at the sacrifice of cost and comfort (when compared to softer compounds.)
I've got over 1400 miles on my Schwalbes ($90/pair), but I used to burn through a $20 pair of tires every quarter back in college.

I get about 2,000-4,000 miles out of my 700x35's (street only).

I assume smaller diameter tires will wear faster as well as knobbies on the pavement, but 200 is far from what I would consider a tires lifespan (unless you're riding in lava).

Actually, Knobbies on pavement could well wear down to slicks that fast. Since they're wearing like that, just run 'em til they pop and replace them with some good street tires!

you can rotate them too and get a few more miles out of em

you can rotate them too and get a few more miles out of em

Be very cautious in how you rotate tires.

see http://sheldonbrown.com/tire-rotation.html for why.