At 3:00PM on a Thursday, it generally takes me exactly 30 mins to get from Teaneck/Englewood to LGA, so I would estimate another 45 mins to get from LGA to JFK. However, I believe a good rule of thumb at that time of the day is to add another 30 minutes of travel time for every 15 minutes you delay your departure from the Englewood/Teaneck area past 3:00PM.