I want to marry my Canadian girlfriend!

Yes. They (and you) are looking at the phrase "up to" and thinking that it is automatic. Also, filing for a B1 or B2 by mail while in US is not the same as crossing the border.

CBP only admitted her for six months. You can phone any POE and verify this. If she was admitted for longer she would have a card in her passport.

Please re-read my post; I've added details.
Quite frankly, your lawyer is agrreeing with the book quite simply because s/he doesn't know any better. Few immig atty have dealings with Cdn immigrants, and are unfmiliar with CBP as opposed to CIS regulations.

How long does the I-539 process take? How soon before her 6 months are up should she file for the extension?

If my girlfriend is here on a B-2 status, what are the best reasons for her to request an extension? What does the USCIS consider a legitimate reason?

She can file up to the day before her six months expire for it to be timely. The process takes 4-5 months. It is not important that the I-539 be approved. The important thing is filing it, and then getting all else done before it is possibly denied. Good guidance is 45 days before expiry.

Most request a B2 extension for health reasons, or request a change to B2 after years inanother status, to wrap-up US affairs (like when a TN loses a job). neither fits her case. There are no 'legitimate' reasons in her case. But she should not say that she is staying "to get married".

Ask your brilliant lawyer to read another book and find one.

On the USCIS web site under the heading "Who May File For Form I-539" it states that a B-2 visitor must file the application for an I-539 extension with the original form I-94. My girlfriend never got an I-94 form and to my knowledge they don't give I-94 forms to Canadian B-2 visitors. Does my girlfriend have to get an I-94 form to file for an I-539 extension, or are Canadian B-2 visitors exempt from filing the I-94 form?

First, even if one did have an I-94, one would *never* send the original, a copy always suffices. One needs to carry their I-94 with them at all times.

Second, as you correctly state, Cdn typically don't get I-94s for toursit entries, so the answers you supply are N/A for the I-94 number. They will know she is Cdn. The date and palce of entry should be supplied.

Thank you AGN!

And you're sure that she will not violate her B-2 status if she stays here past the 6 months and waits for the results of the I-539 form?

Positive. She will be in B-2 "pending" state while you get married.

Rememebr, do not wait for this B-2 to be processed. Get married and file all rreq'd paperwork as soon as you can. She is in a very precarious position in US right now.

I assume you meant not to wait for the I-539 to be processed?

I understand what you're saying, we still have enough time left on her B-2 status to get the I-539 filed and hopefully get everything else done as well. There are underlying circumstances that are holding us up.