She became your common-law partner as soon as you joined your affairs, so Nov or Dec 2008. As such, she should've been declared by you prior to your landing.
She would not have qualified as a common-law partner for the sake of immigration, but she was your common-law partner.
Someone on another page explained this better:
However, another aspect of this is worth emphasizing: there is a distinction between having a "common-law partner" and having a "qualifying common-law partner relationship" for purposes of immigration. I am not sure, but I believe you can be in a common-law relationship even if that relationship does not qualify for immigration purposes, and in particular, I think that relationship begins when you enter into it, even if it will not satisfy the immigration process until after living together in such a committed relationship for a full year. (The fact that certain consequences would flow, including problems with your PR process with no recourse since the relationship would not be "qualifying," does not alter the status of your relationships nor your obligations to report them . . . even if it would undermine your PR entirely, well, you had an obligation to report it.)