marrying Japanese girl in Japan

I am in the United States.
My girlfriend is in Japan.
If I were to go ther as a tourist, and then marry her in Japan, how long would I have to stay before getting a visa allowing me to leave and return as often as I wanted (also allowing me to stay in Japan permanently.) I know that if she were to marry me here in the U.S, it would take her two months to ge a work permit, another 4 months to ge a temporary green card, and up to two years to get her regular green card. I want to know how that process differs in Japan. Thanks for your help!

is your girl friend Japanese, living in Japan or having relatives there? surely she should know the answers or how to find out?

Look into the requirements for a Spouse Visa at the website of the Japanese foreign department: http://www.mofa.go.jp/
Lots of regular contributors here are living on Spouse Visa in Japan and might be able to share their experiences.
In general I believe the following is true (but make sure in any case, contact the Japanese embassy, for instance): you can enter on a tourist visa and get married and then apply from within Japan immediately for a Spouse Visa.

Please check with the Japan embassy in your country to be sure, but I believe what Kappa says is correct.

My non-Japanese spouse was in Japan already with company-sponsored working visa, which we could change to Spouse Visa upon application within Japan. I think that took just one month or so. But the time could vary depending on your situation...

Please note that when you apply for Spouse Visa the first time, normally you get a one-year spouse visa (particulaly if you've just gotten married, this is most likely), and only after a few renewal (you and your spouse have to submit the same kind of documents again for renewal), you will get a three-year spouse visa. After you've been in Japan for at least five years on spouse visa, you *could* try applying for Permanent Residence status (equivalent to Green Card in US I think).

The only thing I can tell you is this. My wife is Japanese and I'am American. Even though we got married in the states first when we came here I had no one to sponsor my Visa, meaning I was unemployed. My Father-in Law had to write a letter to the powers that be, to say he would be take care of me and any cost I accumulated. Basically the note said that me and is daughter would not try to live off the dole.
If your gonna marry this girl you have better make sure that her family is part of your team. I'm lucky my inlaws are awesome and respect me but I did not wave a magic wand to make this so. I asked her father for permission to marry her(came here before we married in the states just for this reason) first and stated my intentions to him.
Her parents were less then receptive at first but after moving here and making an honest effort to adapt and learn the language (I go to school 4 days a week) they came around and saw my true intentions.
Dude your thinking about entering a cross cultural/ineteracial relationship. My wife and I are doing fine now but the first couple of years was a rough road. Be sure your ready.