I must say it all depends on the flight (and some common sense).
If it's a full flight, and if there are other flights arriving at the same time, well, there will be more people in lines.
And there are airline factors. If you fly Japanese carriers, you're likely to be packed with lots of Japanese passengers, including pacakge tours (they have no manners! yuck!). From the US, United and Northwest tend to have more Japanese passengers than other carriers.
Narita is also a hub to many Asian destinations. Many passengers simply transit and do not enter Japan. You may pay attention to the dynamics of your flight once you board. Also, your seat is a big factor when it comes to time. Obviously, if you are seated in First or Business classes, you get off first. Then, even in the coach/economy section, if you manage to get a seat in the front of the section, you'll get to exit early. You save a few minutes there.
I understand that the OP has a Japanese passport. Japanese line at the immigration seems to move faster.
If you were on a full flight and if you checked in early, you may need to wait for your luggage longer than when you are on an empty flight. No rocket science there.
And for the customs, make sure you pick a right line. Stay away from OL-type with big luggage. They tend to be stopped. And anyone with musician-type appearance. They don't like you. They think you have something you're not supposed to bring in to Japan or something.
I've been to Japan 4 times since 2004 and I still haven't asked to open up my suitcases. I just said that I had nothing I could declare! And that I was visiting my family. (And I still haven't asked to show anything upon my arrival at LAX, either.)
But if you are relocating and have separate shipments (besso-hin), be prepared to spend extra minutes. But unless you have items you need to declare or the custom agent feels that any of the items you're carrying may be subject to duty taxes, you don't really have to worry.
Just have your paperwork ready, get out of the plane quick, walk fast, grab your stuff quick, and find a good line at the custom.
Allow a few more minutes if you choose to stop by the bank to change your money (I highly recommend it).
Ask around to find out the JFK flights. If you don't have anyone around you that fly to Japan, try JINA or other Japanese communities. They can tell you what it's like.
I hear United no longer flies direct from New York to Narita. I don't know about other carriers back east.
If you are moving back for good, Japanese travel agencies have cheaper one way fares. Airlines seem to only offer full price one way fares.
FYI - All flights out of LAX bound to Japan go to Narita. No more Osaka flights.
My last trip in February: I took American (AA 169) from LAX. Because of Chinese New Year, the flight was full. But a good chunk of passengers were changing at Narita for Singapre, Thailand or many Chinese cities. Plane arrived at the gate around 5 pm (Terminal 2). I was in the middle section of coach and it was about 5:15 when I got out. Immigration line was not very long, but still had to wait about 10 minutes. Actual paperwork with the officer took less than a minute. He just takes my passport, do his things, and return it to me. Japanese line seemed to be shorter and moved faster. Percentage of Japanese passport holders on American is definitely lower than Japan Airlines (duh). Then the baggage. Because I checked in rather early, my two suitcases didn't come out for about 20 minutes. Rushed to the customs. Took me about 5 minutes including the wait time. I had to take the 6:05 pm bus to Shibuya or I'd had to wait until after 8 pm. I got outside at about 5:55! I rushed to the bus counter, got my ticket. Then I had to look for a public phones to call my sister. Where are they? I had to rush to the end of the terminal. Made a quick call. Then, I saw restrooms right by the phones, I went in super quick. I realized that I didn't have time to stop by the bank to get some yen, but I knew I had about 10K yen with me from my last trip, so I decided to take care of that once I got to Tokyo. I rushed to the bus stop with a cart full of my things. I made it to the 6:05 bus!
