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:

• This person has been living with you in a conjugal relationship for at least 12 continuous months

Note

In this definition, 12 continuous months includes any period you were separated for less than 90 days because of a breakdown in the relationship.

• This person is the parent of your child by birth or adoption
• This person has custody and control of your child (or had custody and control immediately before the child turned 19 years of age) and your child is wholly dependent on them for support

For additional clarification please contact the Canada Revenue Agency: Individual tax enquiries line

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