The buyer has 30 days from the latest estimated delivery date which you should be able to see in your 'orders'.
If they have opened a payment dispute rather than an Ebay case, they do have longer to do that and you would need to have tracking proving delivery.
I would advise keeping your proofs of posting for a longer period - I keep mine for a year.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)
