This subject drives me mad. My chains do NOT stay tight. I have tried everything I know. This is multiple saws and two of them are pretty new and the bars are not worn, they're not that old at all. Chain tension doesn't last anything close to as long as what you are saying above. I use mostly Stihl chains and I am mostly speaking of 60-80 cc Stihl saws that I have been using lately. I am talking bucking big oak logs, 20" to 36" diameter. I will say that a problem is dirt/sand on the underside of the logs, if that matters.
I bet I don't get 30 minutes of cutting without tightening the chain. Run out of gas by then anyway and it's time to fill and also tighten it up.
It's not unusual in cold weather for a chain to be adjusted properly when cold and then be loose when it is warms up.
I've heard of but never has happened to me......that a chain that is tightened up when warm may contract when cold and bend the crankshaft. Maybe an urban myth.
Take a sharpie and mark the bar. You need to know if it is temperature changes or if your bar is really moving in and making the chain loose. It isn't always the chain wearing and stretching.
Make sure that all mating parts of the bar and mating surfaces are totally clean next time you disassemble. It is usually a well lubed area from bar lube.