If you're a permanent resident, the rule is that within every five-year time frame, you have to spend 730 days physically in Canada, with some exceptions. The problem is that when people run up against a situation of aging or ill parents, they end up having to spend protracted periods of time outside the country and then put their own permanent residency in jeopardy if they haven't been able to maintain enough time in the country.
On October 27th, 2016. See this statement in context.