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