Once you get your PR visa in your passport, you will have a time limit to land in Canada. You have to land before the visa expires.
Once you land, you become a PR. In your first 5 years as a PR, you need to spend at least 730 days in Canada to keep it and after that, 730 days in any 5 year period. That means after you land, if you leave again, you can be gone up to 3 years but then you must come back for 2.