kat, i've had a few motors last me over 10k miles, and that's heavily modified,45+mph, but street use only.
once i started racing them, they don't last as long.
dave rust from arrow cycles put it into perspective for me.
the tracks we race on are just under a mile. 2 heat races are 6-8 laps each, with a 12 lap final. plus a lot of practice laps.
one day of racing puts at least 50 miles on my bike, all at full revs, slipped clutch, and flat out in the straights.
in a race, if it starts rattling, clanking, or anything else, i don't care as long as it makes it across the finish line.
most of the time it does. if it doesn't, oh well. since no one's invented a piston rebuilding tool, i need a bottle opener and a lighter to get through the rest of the race day.
and the bottle opener is optional...