You should change it when 12 links are stretched beyond 12 1/16". Beyond that, the drive train will begin to wear very rapidly. The mileage people get out of chains varies, depending on riding conditions, chain type, gear ratio, and riding style, but in relatively dry, clean conditions you should expect to get at least 1500-2000 miles out of a chain...probably more. I've gotten as little as 400 miles out of a chain, but that was in really ****ty conditions. You can measure your chain with a metal ruler, but its easier with one of these....

http://www.nashbar.com/profile.cfm?c...in%2FFreewheel

With proper chain maintenence, a freewheel or track cog should last through several chains...maybe 5 or 6 chains..I always loose track before I wear out the cog.