It's up to the VO. If you're living in Canada, they likely won't require you to get another one because your US criminal status is unlikely to have changed. Further, when you sign your COPR, one of the things you sign is that you have not been charged or convicted - so if you lied and they find out, you are guilty of misrepresentation and your PR status can be revoked on that grounds.
Why every two months? If you ordered one today and sent it in two months from now when you get it, you'd be covered for another year.