how many miles till rebuild?

It's not the miles, its the hours. It also depends on how hard you run the bike and how you take care of it.

Expect anywhere from 10 to 100 hours. I know that doesn't help you but I can't see how you ride and how often/well you clean your air filter.

You have a new KDX, don't you? If you leave your stock jetting as is, you'll have to service the power valves before you service the rings.

Back when all the two-strokes were air cooled, I used to do a top-end job every year. But after having three water cooled KDXs, I decided I was wasting money. Right now I have over 4000 miles on my 99 220, and it still runs strong. But I am doing a top end job this winter.....probably new piston, rings and top end beraring. Plus, of course, cleaning the power valve.