I say "it depends" on your first question. I've had situations where my gates were right next to each other. On other occasions, the gate has been at the other end of the terminal; it's a BIG terminal with LOTS of gates. Can be lots of walking.

I don't yet have Global Entry, but the last several times I've been through MIA, the biggest roadblock has been the lines after retrieving luggage. I would say at least and hour and a half. Mileage varies greatly.