How many days would a person need in order to see the main attractions in Japan?

I know "main attractions" is not exactly the same for everyone, but just use your own judgement about what those are.

How long would one need to be in Japan to see them all?

Like you said, it depends on what you qualify as "main attractions" and also your travel style. If you don't mind rushing through and seeing as much as possible, obviously you'll be able to see a lot more but you'd be sacrificing quality time at each spot.

You could do Tokyo proper in two days I believe. Add a day for each day trip from Tokyo (e.g. Nikko, Kamakura, Yokohama, Hakone).

You'd need about two days for Kyoto and one for Nara. Add a day for Osaka/Kobe.

If you're throwing in Hiroshima and Nagasaki, I'd add 3 more days.

Throw in Hokkaido and that's another 3 days.

In total for all the places I mentioned, that's 16 days and I think that would be fairly rushed.

My dad recently came over and bought a 3-week JR pass. He opted to forgo Tokyo and traveled all around western Japan. He saw most of Nagano, Gifu, Kansai area, Hiroshima, Kyushu, and Shikoku and had a great trip.

A life time?

But really it depends on the traveler, are you one of those people who can visit 5 European countries in 3 days or someone who needs to be at a place to study or to soak it in.

Japan is a large country made up of many islands, and getting from place to place also takes time and money.

I think your question is a bit too broad. Main attractions in ALL of Japan... That's like all the islands. There's a lot to see in Japan... Maybe you should rephrase your question to a specific region or city.

I've lived here for more than 5 years and I've seen about a third of the things that I want to see.

Realistically, about 2 weeks would be the minimum if you want to get a broad look at Japan. Maybe 5 days in Tokyo/Kanagawa/Chiba, 5 days for Kyoto/Nara/maybe Osaka, and that leaves you only a couple days for everything else, maybe Hiroshima or Okinawa or Hokkaido if you can afford the travel expenses. Depends on if you want to wake up at 6am everyday and follow a strict checklist schedule, or relax and take it easy.