Will it eventually say declined or something on the mempool? Or what exactly am I waiting to see if it is stuck and never gets confirmed?
It will eventually fall out of most nodes that have default settings:
Even now it's not in blockchair
https://blockchair.com/bitcoin/transaction/3a88b7e9edb449562f6a62e05e763d3890b7ffb52b5e8d0c21a19a2f1b2bd042
The catch is most pools & explorers keep TX for a very long time since the pools want to have the chance to mine whatever they can and the explorers tend to want to display as much info as possible.
Other then paying for acceleration it's just a waiting game.
But as LoyceV said it really could stay there for a looooog time:
If the sender keeps broadcasting the transaction, there is no maximum amount of time.
Do you know from what wallet the sender sent it from?
-Dave