From my experience, BG check has no limits. It depends on a case's particularities such as the number of people in the application, country of residence, profession, education, amount of time certain government services respond to Canada's information requests, etc. I know lots of cases with over 2-3 years of wait time, where only a writ of mandamus and the consequent court case against Canadian Immigration services helped people to get their PRs.
Country of residence, in my opinion, is the biggest factor, as it is usually the sole thing that decides whether the case will have security screening or not.