start with the rollers- are they all broke? loose?- this will ruin a good sprocket fast!!
#2--- stretch the chain out flat ,and bend it side to side- if it bends too far- get a new one - dont take the chance of it snaping on you -(or someone else)-- wide open -ect...
The mfg's will recommend a change-out at about 1.5% over new pitch for a non-ring chain, and about 1% for a ringed chain.
They state differing time tables even though the ring and non-ring chain are essentially the same thing. The reason is that a ring chain will start to wear once the pre-injected lubricant has seen it's usefulness decline, and once it starts, it will grow very, very fast.
Much faster than a non-ringed chain that can still be lubed.
If you are running a steel rear sprocket, you can probably get by with a chain until it sees about 2% growth.
#520= .625" pitch length
10 links = 6.25" center-to-center of the pins new (chain must be taught)
6.25" x 1.5% = 6.34"
6.25" x 2% = 6.38"