Q: When does my live-in girlfriend become my common-law wife?
A: For family law purposes, the answer will depend on why you’re asking the question.
If you are a parent and you’re worried you may have to pay child support, you’re out of luck. In Ontario, every parent is obligated to support his or her child. Although child support is typically paid to the parent who cares for the child (often the mother), it is actually for the benefit of your child, not your significant other.*
If you’re concerned about spousal support, the answer depends on the nature of your relationship. Under the law, your live-in girlfriend is your spouse if you have lived together continuously for 3 years or longer, or if you’re in a relationship of some permanence and the two of you are the natural or adoptive parents of a child.^
For rights relating to property, unmarried couples are not covered under the Family Law Act. Therefore, such couples when separating are not required by written legislation to equalize net family properties accumulated during the relationship, unless otherwise provided.#
That being said, if a couple have lived together for a long time, there will likely be rights and obligations under common-law doctrines, such as constructive trust and unjust enrichment.
However, because those doctrines are not codified by legislation, whether a spouse is entitled to share the assets accumulated during the relationship will have to be examined on a case-by-case basis.
*Family Law Act, R.S.O. 1990 c. F3, as am., s. 31
#Nova Scotia (Attorney General) v. Walsh,  4 S.C.R. 325
This blog is provided for educational purposes and for your reference. It is not intended as legal advice and should not be regarded as such. The law may have changed since the publication of this article.