You're planning on living in the US, right? She can stay for 6 months, but you have to renew the status just like in Canada. But, the US is WAY stricter about this, and it's hard to do. And whatever you do, don't tell the US border people that she/he is planning on living in the US. They are even stricter about that than Canada is. The renewal you can get in trouble if they think you intended to live in the US the whole time, so be careful. I'd get advice from a US visa forum. I have heard you can ask for 12 months at the border, but I don't know about this. You'd have to have a hell of a lot of proof that you intend to go back to Canada, and that you can support yourself.
If you happen to be getting married in the future, then your prospective spouse can get a k1 fiancee visa which would solve a lot of these issues.
I think within 45 days of the 6 month mark, you fill out an I-539. But, you also need an I-94 which I *think* you can get online, but I am not sure on that, and some people say you can't get this I-94 as a Canadian. The status renewal is expensive too, $290 USD. And, I think it's nearly impossible to qualify for this as a Canadian. That's what I've learned when I was thinking about doing it. I'd be interested to know what other people say.