Month to month might work to see where it will take you, as suggested above. Try it for a month and see if it is what you think it is. Have an exit strategy where you don't get burned.
However, don't let her move in permanently with you. Make sure she keeps her apartment or house or wherever she lives. Over-nighters are OK but not every night (that way it cuts down the chances of her claiming relevant domicile rights).
The advice about no joint bank accounts is on target. Don't sign anything requiring joint ownership.
Last, ask her a lot of questions about who she is, etc. Get to know her real well before you decide. Even check her out on sites that provide personal and criminal checks (It will be worth the fee). It may surprise you.