losing my PR

See appendix A in the guide for "Applying for a Permanent Resident Card" which can be found around twenty plus pages into this pdf document -- there is a chart near the top of a page that clearly sets forth the residency obligation.

There are other sources or explanations of the obligation, but the one I just linked to is as clear as any of them.

Otherwise: just be sure to have 730 days credit for residing in-Canada within any five year time frame and you will be in compliance with the obligation.

Caveat: while there is no talk of it yet (that I know of), I anticipate that the Conservative Majority government will be addressing signnificant changes to the immigration laws in the near future, so any PR spending considerable time abroad will want to follow news of any changes that affect the residency obligation. Certain changes are almost certain to be implemented, such as a conditional PR status for partner sponsored PRs and some enforcement provisions, while others seem quite likely, such as the citizenship residency requirement, and in the course of addressing these I suspect the very lenient residency obligation may very well also be on the agenda.