Usually they arrive at gates G3 or G5 but it does vary. From the farthest gate in G concourse (that UA uses) to immigration is at most 10-15 minutes walk (and that is if you move slowly and skip the moving walkways).

There are no special priority lines (just sorted by EU and Non-EU citizens). Rarely does it take more than 10 minutes unless you find they are on shift change or a couple flights have arrived at the same time. Usually the latter is not a problem because AMS has a few different immigration check points and they tend to open more lanes (eventually) if things back up. As a result no one area backs up too badly. In the past 2 years arrving on a Monday mornign, I never waited more than 20 minutes (except 1 time where it took close to 45 minutes and I have to wait on the stairs before I could even get into a line!).

Being in C you'll pretty much beat all the other UA passengers to the immigration lines and zip right through. After immigration, pick up your bags (if applicable) and then walk through customs (which doesn't have lines) and then you'll find yourself in the arrivals hall.

If you have to buy a train ticket you'll find that line can be A LOT longer than the immigration queue.

All told you could get from the gate to Amsterdam Centraal (via train) in about an hour.