To get back on topic... it depends entirely on how heavy you are on the twisty tube. First sprocket I got 16k miles (slightly over 25000 km), by not being a hooligan. I replaced it because I had a couple of long rides coming up and was replacing the rear tyre anyway, but probably could have nursed another couple of 1000 klicks out of it as it wasn't too badly worn.
The new one has only got 9500 miles (15k km) and is starting to look a bit wave like on the teeth but I have been riding more agressively than last year. Also when I replaced it I used a non ktm sprocket, which might have something to do with the wear.