I am flying to JFK from Athens. Arrive 6 pm—12hr layover connecting flight to LA. Can I have bags rechecked immediately after going through customs or have to haul them around for 12 hours.

Recheck as soon as you arrive only about 5 minutes

If you check your bags all the way to your final destination, you can drop them off just behind customs. This assumes your connection is on the same airline

Re check them and it is fast and easy.

You can recheck as soon as you get there. You shouldnt hold ur bags long