This all depends on where you're going and how you'd like to leave the airport. If heading into the city or out to Long Island and you'd like to take public transportation; you can get the air train from JFK. The Air Train is a monorail system that goes in a loop and is $5 on a metro card (that you can get at the terminal). The Air Train will take you to Jamaica Station (about 15-20 minutes) and from there you can either get a Subway (metro card) or the LIRR (Long Island Rail Road-purchase ticket) to NYC or Long Island.
If you're going by car/taxi it's best to organize in advance (unless you're headed to NYC and then you can jump in a taxi-but not inexpensive) to get the best rate possible.
Customs, like anywhere else, can be a quick stop or an hour plus depending on how many people are on that morning and how many international flights arrive at the same time.
Safe and happy travels. -Stacey firstname.lastname@example.org over 7 years ago