http://www.cic.gc.ca/english/information/times/canada/cit-pr...
What this means is that in theory, someone could land as a PR one Day 1, file the application exactly 3 years and 1 day later, and then leave Canada and go sit in a hut in Guatemala while the application is being processed. [1] After it is ready for the test/oath in 2-3 years, applicant flies back, declares a desire to live and die in Guatemala, and still gets his Canadian citizenship, a shiny new Canadian passport, and leaves the country. And all this is 100% legal, at least as per Canadian Citizenship Law as of 2013.
[1] Unfortunately, C&IC will only correspond to Canadian addresses, so the applicant does need a reliable mailing address in Canada
Good discussion here: https://secure.immigration.ca/forum/forum_posts.asp?TID=8848... (see a post by dpenabill toward the end of the page), and the entire thread is good as well.
There is ambiguity about whether it's a rolling 5 years or whether it's 5 year cycles based on the date you first receive your PR status. To be safe, it's usually best to assume modulo 5 from the start of your PR status.
In the example above, if you filed the citizenship application 3 years and 1 day later, the maximum you can stay in your Guatemalan hut is just shy of 2 years, or else you risk losing PR status and jeopardizing your Canadian citizenship application.
[1] http://www.cic.gc.ca/english/newcomers/about-pr.asp#keep_sta...
(Having no dog in this hunt as I'm not Canadian nor do I have any claim on Canadian citizenship or permanent residence, much as I'd like to.)
The estimate is based on "processing time from receipt of application to final decision for 80% of cases".
The rather excellent British Expats (not just for Brits!) forum users keep track of the various processing times of Canadian immigration applications.
http://britishexpats.com/forum/forumdisplay.php?f=33
British Expats forums is a great resource for anyone trying to move within the English speaking world.
I got my citizenship about ten years ago (I only know because I'm on my third passport), about six months after applying and taking the test. I had been resident for just under four years at the time.