JFK to Manhattan on a good day can take an hour. Are you connecting to an international flight? Then you would have to be back at the airport at least 90 minutes before your departure. I wouldn't risk going to the city during the holidays because it is nuts near Rockerfeller Center. You can hardly move an inch. It is a cool sight to see if you never have seen it during the holidays but if your income flight is delayed slightly you would be cutting it on the close side in my opinion.