I don't think this form has an expiry date, as it's something equivalent to a marriage certificate that you do once and then can use that same document indefinitely as long as you stay common-law. You only need to "declare" something once. I'm not entirely certain on this though.
Considering the form is not even required anyways for spousal sponsorship, I really don't think it matters. Absolute worst case scenario is the visa officer simply asks you to do a new one sometime during the process.
Note even the form itself is dated 04-2015, meaning it was last updated April 2015.