If you are a permanent resident of Canada and not a citizen of Canada, your Permanent Resident card (or PR Card) is the document that confirms your status in Canada and that allows you to re-enter Canada on arrival from a foreign country.
Normally, PR Cards are valid 5 years. You should apply for renewal if your card has expired or will expire in 9 months. You cannot apply earlier. When you apply for renewal, the Canadian immigration authorities will assess if you have lived in Canada for at least 730 days in the last five years.
If you have not met these requirements, you still stand a chance to renew your PR card if the officer is satisfied that you fall under one of the exceptions to the 2-year-rule.