It should be under an hour really, and hopefully shorter, the biggest factor is whether your aircraft is on time and/or the movements of other aircraft, about which you won't be able to do very much. Clearly a rapid passage / brisk stroll to CBP will help since it will take a while for the main WT cabin to get to CBP. If you have been to the USA on ESTAs before then reading up on the kiosk process / videos on the TSA website will perhaps help, but since thousands of people get through CBP every hour without watching videos, it probably won't make much difference.