Re: Anil R, my experience w/ JFK (which is extensive - I've travel around the world 10x from there, so here's what I can share:
1) You should not have to go through customs if you are flying on. Typically, your bags should have been forwarded through, unless you instructed them otherwise (or if that's a new policy), Regardless, you should be able to get off the plane in about 15 minutes, unless there's weather or air traffic, and you have to 'stay in the pattern' which is fairly frequent. But once you are at the gate, after deplaning, you walk about 1/2 mile to customs, and queue (which could be fast, say 5-15 minutes, or slow - I once had to wait 45 minutes.
2) You then go to baggage claim (make sure you know which one - this can get confusing, but they have digital signs working most of the time, so If you don't see it within 3 minutes, ask! Then, getting you bag is usually pretty fast, say 10 minutes, up to never if they lost it (then you have to go to Lost Luggage..); But a typical LONG wait is 30-60 min.
3) walk to the first outside lane where buses stop, and hop on any blue bus (the parking lot buses work, as well as hotel shuttles usually) and make sure they will stop at Terminal 8. The parking lot buses should be stopping at every terminal, and these buses (long. blue buses saying "Lot C" for going to the long term parking lot Have to stop at every terminal, so you usually see one of these every 5-10 minutes stopping (they're free - hotel shuttlrd too, . To get from T4 to T8 is about 20 minutes (5 minutes per T-stop). If you have to bring your luggage, you stand in the security line, which will take 15-30 minutes.
4) This is if the Gods are with you and no problems arise, So the BEST time from getting off plane 1 to boarding plane 2 is going to be 2 hours. Murphy's law says you may run into a snag, so just be prepared to deal with it. Trying to take a taxi from T4 to T8 will be a ripoff, even though the signs say otherwise.
I hope this helps, and that the gods of travel keep Murphy at bay; If you can "check your bags through on to Amsterdam" then by ALL MEANS DO SO!
And please, some the best Hawaiian or Thai stick you can find. I want to go, just so I can be stoned in peace for a day or two.
Happy Holidays!
JFK Terminal Timing from T4 to T8