Yes, you can marry her here in the US and sponsor her Green Card.
But after F1 expiration she will have to leave the US and wait a few years for the green card, unless octopus is eligible for citizenship sooner than that, or she has another means of staying in the US legally (e.g. H1B visa).
The wait for spouses of green card holders is almost 3 years, and the spouse is not allowed to remain in the US while waiting out those 3 years, unless they have some other kind of visa or status separate from the marriage (e.g. F1, H1B).Ok. If I don't take up the citizenship, marry her, and stay in the U.S. with her as LPR. How long is it gonna take for her to get GC ? How many years?