If you are asked by a borrower when a requested item will be available, the only true answer is ‘When you receive Hold notification.’
There are so many variants on due dates (if the requested item is out), collection times (Storage, IBDS), time in transit, schedule for changing New Book displays, searching for missing items and processing time for all of the above. If pressed for a date or time of day then make it clear that it can only be a guesstimate.
If you have any reason to believe that fulfilment of a request is unreasonably delayed, take details of the borrower ID and details of the requested item then advise them that you will follow it up and let them know if there is a problem.
Helen