The Marital Status of your tax return is filed according to what it was "on December 31st of the tax year". However, you only need to claim common law status for 2018 if any of the following situations apply to you:
- You have lived together with your partner for more than 12 consecutive months,
- You have a child together, either related to you by blood or through adoption,
- Or either of you have primary custody of a child under the age of 18.
Basically this means that CRA considers you to be in a Common-Law relationship as soon as you move in together if there are any children residing with you.
https://turbotax.intuit.ca/tips/marriage-and-common-law-relationships-knowing-the-difference-455