Yes, you should get easily twice that mileage, mine's at over 30,000 now & shows no sign's of needing replacement any time soon, most of it has been done well loaded, often 2 up & not hanging around. make sure you're not running too tight, when you adjust, over tighten the primary chain, then use the adjuster to pull the gearbox forward to loosen the chain, then tighten the gearbox bolts to hold it all in that position. then you can adjust your final drive chain. Adjusting in this way, the adjuster bolt holds the gearbox from being pulled backwards by whip or snatch in the final drive chain, which then tightens the primary chain. Hope that all make sence.
Regards, Tim