Go by the delivery date which was written on the original listing. If the seller has another identical item for sale and you look at that listing, it will show the delivery date if you bought that item today.
Having said the above, if your original listing had a 3 day delivery time, taking into account these are just eBay's estimates and a tad optimistic, but if a reasonable time passes that estimated date with no item, you can open a case for item not received for a full refund.
@gwc1gareth