if you're already in canada and you want to stay to become commonlaw, it's much better to apply for an extension of stay online with IRCC. if you leave for an extended period of time, then you can not count that time towards your commonlaw status. you need to be able to prove 365+ CONSECUTIVE days of cohabitation. if you leave, and you haven't reached commonlaw status yet, you'll end up needing to start from scratch.
ask scylla already suggested, it's best to apply for the extension no more than 30 days before your current stay expires.
it's acceptable and usually recommended to ask for a 12+ month extension. While they can always give you less than what you ask, they will never give you more. if they don't give you enough time to make commonlaw status, then you would want to apply for another extension down the road.