As per our TurboTax FAQ:
Facts Every Canadian Needs to Know About Filing Coupled Tax Returns
Once you are common-law, to be considered common-law, two people must live together in a conjugal relationship for 12 months or immediately if you have a child, then you must file as common-law.
As per the Canada Revenue Agency (CRA):
Living common-law means that you are living in a conjugal relationship with a person who is not your married spouse, and at least one of the following conditions applies:
In this definition, 12 continuous months includes any period you were separated for less than 90 days because of a breakdown in the relationship.
For additional clarification please contact the Canada Revenue Agency: Individual tax enquiries line
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