Is this reasonable?
As a seller, it's important to make yourelf familiar with eBay's money back guarantee to buyers.
eBay allows buyers to open a case for a refund within 30 days of the date of delivery. So yes, the timescale is reasonable. What counts is the buyer's reason for wishing to return it, specifically whether they have just changed their mind about wanting it, or state that it is not as described. You didn't even mention which!
There is a link to the money back guarantee policy in every listing, or click here: https://pages.ebay.co.uk/ebay-money-back-guarantee/