I assume you know that when this happens, you usually can call AA and have them change your flights, for no charge, to ones that work better for you. That said, I have transferred from JFK to LGA several times this year (flying AUH-JFK/LGA-BOS), and routinely make the transfer in around 75 minutes from when the door opens at JFK until when I'm through security and at the gate at LGA; actual travel time in the taxi/uber has been less than 30 minutes each time. (Note, however, that AUH is a pre-clearance station so I do not have to clear immigration and customs upon arrival, and I have traveled without checked luggage.) In my case, the 240-minute minimum connection time requires me to book a 260-minute connection (9:40am scheduled arrival at JFK; 2:00pm scheduled departure at LGA), but I routinely make it to LGA by 11:00am and easily make a 12:00pm departure (and would make an 11:00am departure if AA didn't routinely close the doors of its LGA-BOS shuttle flights 10 minutes prior to schedule departure time).