If you're early off the aircraft, even without GE CBP at TBIT is pretty quick using the kiosks. Not sure what T6 security will be like early in the morning, but without Pre, I'd budget 15-30 mins. So with no checked bags, I'd say 90 minutes is probably safe. Wouldn't want less than that. On separate tix, 2 hours would probably be my minimum. Even more to really be safe.