There is bus service, but it is about as convenient as the aforementioned rail option as it does not depart nonstop from the airport. I would rent a car. Time varies on time of day, as in avoidance of Philadelphia's outbound rush hour (4-630pm). Assuming no traffic, you can do PHL rental car pickup to Atlantic City proper in about 90 minutes inclusive of time at the car counter. Distance is ~70 miles/113km. Google says 65.2 miles, but add on some for leaving the car rental property, etc.