there is no fixed time for it. 72 hours is an approximate time.
here is how it works:
when you send a transaction out, bitcoin nodes receive it and keep it in their memory (remember it) called memory pool (mempool) and the default setting by bitcoin core is to keep it for up to 72 hours and then drop it (forget about it). but it is not enforced to do so, that time can be changed to shorter or longer time and not all nodes have same settings or run same clients.
Yeah no, I sent a guy some btc and he claims he dident recieve it. So if it is stuck, will I eventuelly get it back? Or will it continue to send the btc to him when the miners choose my block?
it is bitcoin so you can not claim you didn't receive funds! everything is out there for everyone to see, go to a block-explorer and search his bitcoin address you can see if he received it or not.
and being stuck can have many reasons, but usually is because you paid a very low fee but there is still a possibility it goes through.