Johan, we run our two strokes typically a year to a year and a half on a topend, usually about 2000-3000 miles of singletrack racing. We are all mostly expert/A riders. Figureing about 20 to 30 miles an hour average, that is around 100 hours of racing. Our 4 stroke buddies can sometimes go a little longer, but boy, if you run them too long without maintenance, the catastrophic failures are really expensive, in the $1000+ range.
We are all very anal retentive about air filter maintenance, gentle warmup, and running freshly mixed high quality synthetic oil premix. I have owned a YZ426WR and an XR650R (four strokes), and sold them after a coupla years of hard riding on their original topends, but kinda had my fingers crossed during their last 6 months, worried something bad would happen and it would cost me an arm and a leg to replace parts. Plus, I got to hate changing 4 stroke oil, and checking/adjusting valves every 600-1000 miles of riding.
I plan to continue racing two strokes as long as they continue to sell em!
My major expense in riding is fresh tires.