Each complete link should be 1" long.
Measure out 12 complete links on the chain.
If it is longer than 12-1/16" then it probably needs replacing.
If it is longer than 12-1/8", then you may need a new sprocket and cog as well.

Oh.. and I love the KMC 510HX
Its not the quietest 1/8" chain, but its pretty effing strong.

EDIT: corrected fractions!