FYI... chain stretch is a common term, but really it is your chain 'growing'  The pins and bushings inside the chain itself wear and that is what makes the chain become longer.

Like I said before, just make sure to buy a good quality sealed chain (O,D,X.W,etc...) and regular maintenance will yield the longest life..... also replace the chain & sprockets as a set.  Also, don't just depend on WD-40 to 'lube' your chain.   WD-40 refers to Water Displacement version 40.... good for getting water out of the chain, but doesn't have lubricating oil required by chains.

Like Throttle Therapy said, just watch new chains that are installed and keep it adjusted to specs.

Measure 21 pins on the chain.  It should not be > 319.4mm / 12.57"

Chain Slack - 25-30mm / 1.0 - 1.2" between sprockets

Good Luck.