Once she marries you she can apply for permanent residency (green card). She can adjust her status to permanent resident without having to leave the U.S. since she entered the U.S. with her F visa.
There is no minimum length of marriage necessary. However if you have been married for less than two years at the time you are granted permanent resident status, your green card will only be valid for two years. This means that your soon to be wife will be granted a 2 year green card and will have to apply to renew it within the 90 days prior to it expiring.
You should speak to an immigration attorney first, if possible, just to make sure there will be no other issues.
Once married she can apply for adjustment of status to permanent residence based on a marriage to a US citizen. Within 2-3 months she'll obtain a work permit and within 4-5 months of her first filing for AOS, her actual "green card".
Since she was lawfully admitted to te US in F-1 status everything will be handled by USCIS here and she will not have to leave the country to get her green card
Behar Intl. Counsel 619.234.5962 Kindly be advised that the answer above is only general in nature cannot be construed as legal advice, given that not enough facts are known. It is your responsibility to retain a lawyer to analyze the facts specific to your particular situation in order to give you specific advice. Specific answers will require cognizance of all pertinent facts about your case. Any answers offered on Avvo are of a general nature only, and are not meant to create an attorney-client relationship.
I agree with the other attorneys here. Your fiancée will be able to adjust her status. You will need to file a petition with USCIS to sponsor her and show you will be able to sponsor her. You will also be interviewed at the local USCIS Field Office. I recommend you consult with an attorney before you begin to learn more about the process. Good luck!
Once you you marry her, she can adjust her status to permanent resident. If she wish, she can continue to study her field at her current school while the case is pending. There is no minimum time period. However, you have to submit strong supporting documents to prove that the marriage was entered into in good faith.
This answer is of a general nature and should not be relied upon as final, nor is it intended as legal advice. Consult with a qualified attorney before making any legal decisions. Gen Kimura, (832) 247-6932.