Available for pickup- buyer not responding

I sent out a package that has been marked “available for pickup”. It got marked on the 2nd so I’ve messaged the buyer twice to let them know. They’re not replying but have been active on Depop. I have a feeling they won’t be picking it up. They’re a first time buyer, no reviews, and sent a lowball offer which I accepted anyways.

How long will the post office keep the package before sending it back to me? Has anyone else had this issue?