Most stock chain/sprox are done at about 20k.
I'd replace the sprockets too. A new sprocket's teeth are flat on top. As they wear, this flat spot gets smaller and smaller and becomes pointy, at which point it starts to curve in the direction of the torque. This creates more friction/heat wear on the the chain.
Buy good sprockets not the ebay cheapos. I've had very good luck with Supersprox, they're not too expensive (front and rear were about $100), and the rear is an aluminum/steel hybrid. I've got about 10k on them and they still look brand new.