You do not need to disclose that your fiancee plans to marry. Apply for the visitor visa soon, as it can take 30 or more days (and will be valid for the duration of her passport, typically). This will also allow you the opportunity to find cheaper flights, usually. She will need an invitation letter from you, and you should provide proof of income with that letter. She should state she is coming to visit her boyfriend.
When she enters Canada, she may be given a date to leave by. If they give her a date, it will be written on the stamp they will stamp in her passport. If they do not write a date in, then she has 6 months in Canada (assuming your avatar image is where she is from, as well).
If you are intending on applying for a PR for her within Canada, then you should also consider applying for the Open Work Permit (OWP) that is afforded to people applying in Canada. It's an extra $255, but it will grant her status in Canada, while they process the PR application -- the PR application does not grant any form of status until approved. So she will have to maintain status by another means (OWP is the best way).
Where do you plan to marry? In Canada or somewhere else?