Out of the blue we got a Chase credit balance check last week - validity is 365 days. The credit balance was issued in 2nd week of March due to a canceled Aeroplan award which taxes were paid with the Chase Sapphire Reserve card. It seems Chase automatically issues check on credit balance on 2nd statement cycle if the card still has credit balance (no need to be the same amount as the 1st statement cycle).
I would think AMEX would at least give 180 days on credit balance refund automatically issued by the system. The chat rep should be able to give you the answer.