no chain checker, generally go by overall elongation. 12 links should measure 12", measured with a standard scale (ruler). i'll change a cassette about every 3rd chain, or when the shifting gets sloppy.

some chains, like campy will not elongate much though, the rollers will wear, and the chain needs to be tossed based on roller play.

generally, in semi-dry conditions i'll get 2500 miles out of a chain. less over the winter when its gritty and wet out.