You must be in Canada 2 years out of every 5 to maintain your PR status. So that means that you can stay outside Canada for up to 2 years after landing, but then you wouldn't be able to leave Canada for 3 years upon returning--not even for one day.
Think of it this way: The clock starts ticking on your landing date. If on the 5 year anniversary of your landing date, you have been outside of Canada for 3 years 1 day, you lose your PR status (actually they calculate by # of days, so you should look it up on CIC's website). I would assume you can also be deemed to have lost your PR status if anytime within those 5 years, you have been outside of the country 3 years one day, as there is no way you can fulfill the 2 year residency requirement after that point.
Another way to think about it: count back 5 years from the date you show up at the border or the airport in Canada. If during those 5 years, you have been outside of Canada for 3 years 1 day (following you initial landing date), you have lost your PR status.
There are some exceptions, which mostly have to do with working for a Canadian government entity outside the country. You can get more clarity on CIC's website.