Immigration is what takes the longest when landing at US airports. We all know that. We also say to factor in an hour in total to get out of the airport.
Well, I've just found this - http://apps.cbp.gov/awt/ - Select your airport/terminal and choose seasonal average, and select the day you're landing, it'll give you an average of how long people waited, and the shortest/longest too.
I now know that my wait time on a Friday in September, getting into the American Terminal are
Noon to 1 PM Min - 16 Average - 22 Maximum- 36
No guessing needed now. Bear in mind that this doesn't factor in how long it'll take to get your luggage, but you will be able to see how long this element of the process it.
Hope it helps