She is allowed to stay as long as you guys can maintain her legal status. If you guys apply with an inland sponsorship, she will maintain implied legal status as long as she submits an open work permit along with the application.
If you guys apply outland (she can still stay with you, but she will not be able to work) she will need to keep extending until she gets approved and "lands." She can ask for a visitor record upon arrival in Canada at the border, which may grant her a longer term stay. For example, she may get one year instead of 6 months. But be aware that if she leaves, the visitor record or previous stamp in her passport does not mean she is guarenteed entry until that time. So if she leaves Canada, she will need to ask for another visitor record, or you will have to apply for another one online before her legal status runs out.