The US doesn't have exit immigration or customs, departing the country is the same procedure (airport-wise) as taking a domestic flight. Same security and often the same terminal.

I rarely show up an hour before a flight (including international) and I've yet to have an issue. Even United says that an hour is sufficient for international flights, even less if you aren't checking bags and can print your boarding pass at home:

http://www.united.com/web/en-US/content/travel/airport/proce...