That depends on the reason. If the buyer is claiming that the item is not as described, they have 30 days from the delivery date to file a claim with eBay or 180 days from the sale date to file with Paypal.
Returns for buyers remorse are dictated by the seller's return policy, so in your case you would not have to accept the return. You should be aware, however, that in a case like that the buyer will often turn buyer's remorse into not as described to be able to get a refund.
