Using an elite check-in line, I've never planned to arrive more than about 90 minutes in advance of the flight and that is plenty of time. I take the train to NRT so traffic is not a worry.

If the lines are huge at outbound immigration (the only place that is a problem typically) and time gets close, staff people circulate calling out the flight number and hurry you through, but you might have to watch for them as sometimes they are not very vocal.

The only time when this would be wrong are holiday times like Golden Week. Never been at Narita at the start of Golden Week, but I bet it is scary.