I've seen some big mileage numbers from a few people, both here and elsewhere. 30,000+ miles.
As long as you're being religious with maintenance and upkeep, it should run a good, long time.
Just keep good oil and gear lube in it, and change it regularly.
Check the spark plug at regular intervals, and replace in when needed.
Check and adjust the valves when they need it.
Also, make it a point to check for loose bolts and loose wire connections as often as possible. That can save you all kinds of headaches down the road.
Take good care of the scoot, and the scoot will take good care of you.
Don't take care of the scoot, and it will take a huge dump on you in return!