I think the answer is here: http://www.cic.gc.ca/english/resources/manuals/op/op02-eng.pdf , under section 5.36/5.37. It seems like once you live together 1 year... it doesn't matter what happens after that and you will always be common-law AS LONG AS you can prove you're still in a relationship in general.
5.36. How can someone in Canada sponsor a common-law partner from outside Canada when the definition says “is cohabiting”?
According to case law, the definition of common-law partner should be read as “an individual who is (ordinarily) cohabiting”. After the one year period of cohabitation has been established, the partners may live apart for periods of time without legally breaking the cohabitation. For example, a couple may have been separated due to armed conflict, illness of a family member, or for employment or education-related reasons, and therefore do not cohabit at present. Despite the break in cohabitation, a commonlaw relationship exists if the couple has cohabited continuously in a conjugal relationship in the past for at least one year and intend to do so again as soon as possible. There should be evidence demonstrating that both parties are continuing the relationship, such as visits, correspondence, and telephone calls.
This situation is similar to a marriage where the parties are temporarily separated or not cohabiting for a variety of reasons, but still considers themselves to be married and living in a conjugal relationship with their spouse with the intention of living together as soon as possible. For common-law relationships (and marriage), the longer the period of separation without any cohabitation, the more difficult it is to establish that the common-law relationship (or marriage) still exists.
5.37. When does a common-law relationship end?
A common-law relationship is deemed severed or ends upon the death of one partner or when at least one partner does not intend to continue the conjugal relationship. Again, the facts of the case must be examined to determine if the intent of at least one partner is to stop cohabiting in a conjugal relationship