There is no exact time limit set for a buyer to collect a paid for collection item,  but after 7 days if no contact can be made,  you can send them a polite message that,  if you do not hear from them in ** days you will deem the item to be unwanted, and will cancel the order,  which will fully refund the payment made.

If this should happen cancel the order citing ' Buyer Requests to Cancel' .  This fully refunds them,  and your seller performance is not affected.

You can then relist the item.

Personally I would block them from returning and possibly going through this rigmarole again.

Add their ID here:

https://www.ebay.co.uk/bmgt/BuyerBlock?

@muddybilluk