After how long does a check become non-negotiable?

I issued a $250 check 14 months ago. Because of a dispute with the payee I put a stop on it which I had to renew after 6 months. I was ready to pay the $18 to put a second stop or open a new account when the teller said after 6 months the check in non-negotiable anyways so I shouldn't worry. So I didn't.... Well, 14 months later this person was able to cash the check. He'd refused to send it back claiming he'd destroyed it. Yeah right. And the bank won't do anything.... I was told that the ';little people'; (cust. serv.reps?) at the bank sometimes are ';not really knowledgeable,'; so the bank is not responsible for that teller's bad advice. How's that.