How long before a non-debited Visa Debit transaction expires?

born_again

Offline is very rare now. Mainly a back up.
Even contactless have to be authorised, or they are declined. 

Car boots using handheld will all go for authorisation or be declined.

Best guess on yours, a loss of link in connection. So may never or got through. Or someone has not run a tape at merchant bank of retailer. Again happens quite often.

You could ring your bank & ask if they can see the authorisation for the transaction. Depending on how long ago it is, might need someone to access security system, which front line staff usually do not have access too.