Chain Replacement,

NOW! What Portis said. IF you run a worn or stretched chain you will mess up your sprockets.
Chains are cheeper than sprockets.
As stated above, nobody knows how long your chain will last, Load, dirt and lack of lubricant will all be factors. Keep it clean and lubed. Like the man said, measure it, but get rid of it if it is stretched.
Right on Portis.