Concorde has an OFFICAL checkin deadline of 30 minutes. HOWEVER, JFK can be a mess and getting through security etc can be a pain. There is a premium class line and BA staff are often helpful but to keep it safe I'd still allow at least 90 minutes from entering the airport terminal. If you are staying inside Terminal 7 it will be pretty easy, if you need to change terminals then keep in mind that JFK is a zoo.
If you have the chance, change the incoming flight to arrive earlier so you can enjoy the Concorde lounge and get the "full experience".
Enjoyr your flight!!