I would say a couple of days from when it showed as being received. If UPS/ FedEx (whomever you used to ship it) has the number of days listed that you are allowed to report hidden damage, then I would say it should be based on that less a day so the seller has time to file a claim. If it truly isn't working now, but was when you shipped it, then it possibly got dropped or somehow mishandled during transit. It would not be fair to the seller to have to eat the cost of repairs when the seller would have missed a chance to file a claim against the shipper. I'm sure there are a dozen more scenarios for why it isn't working now, but this is what popped into my head.
Hopefully, in your case, it is as simple as him not doing something correctly.
Hopefully, in your case, it is as simple as him not doing something correctly.