When does seller's responsibility end?

I'm sure this has been discussed before, but a search could not find it.
I sold a Naim DAC (mentioning this because Naim is notoriously reliable) to someone on November 26, shipped on November 27, 2012.
Did not hear anything one way or another until December 19, when seller emailed to say one USB input would not work with his ipod. Well the input worked for me when I had it. I asked him to be sure he was doing everything correctly, including checking for bad cable. I am not sure the input really does not work.
No word again until December 25, when he asked if I would pay for repair. I answered no, because I felt the time period had been too long and my responsibility was over.
Now he has initiated a dispute/claim against me on Paypal. They have put my account in the negative for the amount of DAC until they resolve it.
Sorry for the long post, I just wonder if there is a consensus on how long a seller should be responsible for "sold" gear. I say a week, maximum.
What say ye?