As others have said, an undamaged chainsaw chain doesn’t actually stretch as such. What it does do is two things.
1. A new chain loop that hasn’t been used will get longer after it’s first use, this is because as you run it in one direction, all of the play in the rivets will be pulled in one direction effectively making the chain loop longer. Say you have a chain loop of 70 links, and each rivet at each link has a clearance of 0.25 mm. (I.E. one eighth of a millimetre all round) then once you have pulled all of that clearance space to one side, your chain will be 17.5mm longer, that’s around three quarters of an inch. All of that initial “stretch” will usually take place in the first half an hour or less of use, that’s why you always have to keep an eye on a new chain and keep adjusting the tension as appropriate.
2. As a chain wears, that clearance around the rivets will get worse and the chain will continually get a little longer every use. The difference each use is small but steady. With an appropriately lubed chain and a decent unworn sprocket that wear will take some time before the chain is worn out (you’ll probably have sharpened the teeth away before it wears out!) - but running a chain even for a short time without lubrication will both overheat it and cause enough wear to potentially ruin both the chain and the bar.
There is a third reason why a chain might stretch, thermal expansion, but with a properly lubed sharp chain, a well maintained bar and a good, unworn drive sprocket, that shouldn’t happen enough to make much of a difference. However if you do overheat your chain don’t tighten it up while it is still hot. If you do it will contract as it cools and could seriously damage your saw.