I've been through immigration and past customs (with only carry-on) in 5 minutes. I've also been stopped by customs and had it take 30 minutes. It also depends on the flight and arrival time. Once out of the secure area, it is less than 15 minutes to walk to the UA domestic terminal, clear security (usually; again, day and time), and get to your gate. I've gone less than 20 minutes from aircraft door open to my domestic gate.