If the chain is worn, or "stretched" to it's limit, then "distance" is irrelevant, it's just an average amount of life a chain will have. How hard you ride, and how clean or dirty you keep the chain, will affect its period of service. If the chain is still ok, then how far it's ridden isn't necessarily a hard-fast rule, it just puts you in the ballpark.
And, as stated above, I'd go with measuring it, versus using a chain checker. 12" on a ruler or a tape measure is going to be 100% consistent, unlike the various models of chain checkers.