I also usually target to arrive at Narita around 2 to 3 hours before my flights. Usually this is overkill, but it gives me the chance to catch a bite to eat and relax in the lounges if all goes smoothly.

On the occassions when NRT security/immigration is really backed up (sometimes well over 1 hour in line), it really helps to have the extra minutes so you don't worry about missing your flight, particularly if Cathay Pacific likes to board early.