Chains don't stretch, but wear makes them get "longer". Measure 24 half-links (each inner and outer is a half link). A new chain will measure 12" exactly. 12 1/16" is marginal. Anything much over that is seriously worn. If the chain is badly worn, it's almost a sure bet that the cassette (small cog set) needs to be replaced too.
http://sheldonbrown.com/chains.html#wear
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