Amazon - third party seller / refunds and returns timeframes

We sell grow your own mushroom kits, so by nature there are instructions to this and are often not followed etc

having tried to read amazons policies, where do we stand with people claiming there to be a fault and with timeframes

often what i am finding is customers say it hasnt worked, some months after the fact without reaching out for advise etc

at what period are we able to say, too much time has passed and we are not entitled to offer a replacement / refund?