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Before I answer I just want to confirm that you are solely asking about the law in Canada. I cannot answer about the law in the US.
She will need to apply for and obtain permanent residency status.
This will be easy so long as you wish to sponsor her as your spouse.
And the application process should be started when you are getting closer to planning the move. It can take up to two years for the application to be approved.
You will have to show that you are moving back to Canada yourself.
Here is a link to the information that you will need:
http://www.cic.gc.ca/english/immigrate/sponsor/spouse.aspAs a visitor she can visit for up to six months at a time but if she keeps leaving and coming back at some point her visits will be limited in duration and eventually she may be prohibited from entering as it will appear that she is living in Canada permanently as a visitor.
Let me know if you need any further clarification.