How long can you stay in Japan?

Question:

Without a visitor visa.
With one.
How to stay 'forever' (like 10 years for example)?
What are the consequences for not leaving when your due?

Answer:

Okay, to answer your questions...

Without a visitor visa?
You can't even be in Japan without some kind of visa. So no time without a visitor visa.

With one?
You can stay 3 months.

How to stay 'forever' (like 10 years for example)?
You will either need to a work visa (I am on one now) and as long as you have a job you can keep renewing your visa indefinitely. I am going on my 7th year. My first year was a one year visa, my second time renewing I applied for a three year. And then I applied for another three year. Another way to stay is to marry a Japanese national (which I have done and will switch over my visa next year). You have to keep renewing your spousal visa, but that is another way to stay as long as you want.

What are the consequences for not leaving when your due?
Immigration (if they find you) will stamp your passport as an overstayer and then you won't be able to re-enter Japan again for so many years once you are deported.