A buyer has up to 180 days to open a dispute for non receipt of item OR item received but not as described OR if they funded their paypal payment via a credit card then they could initiate a chargeback via their card issuer.
You really need to read up on the many factors all listed in Paypals seller protection so you can risk assess your transactions. Legal (bottom right of Paypals pages) > user agreement > seller protection.
Advice is voluntary. Kudos / Solution appreciated.