According to a study commissioned by CanLII and released today reported cases have varying "life spans" and cease to be important — as measured by their citation in other judgments — somewhere between three and fifteen years. The exception to this are judgments of the Supreme Court of Canada, the average "time to failure" of which is a whopping fifty years.
Citation Analysis of Canadian Case Law by Thom Neale is a full-on informatics study that:
uses simple statistical and functional analysis in conjunction with network analysis algorithms to examine the network of Canadian caselaw using data supplied by the Canadian Legal Information Institute (CanLII). Seeking to explore three basic questions, the study describes the database coverage of CanLII along with that of two commercial vendors and juxtaposes that information with the number of citations to cases decided by courts within each province each year. The study then uses analysis of time-series network rankings for each case to determine 1) the age at which cases in the network typically cease to be important, and 2) what characteristics define those cases that continue to be important despite the passage of time.
Although, as stated above, the average life span of all SCC judgments is fifty years, currently (and perhaps at any given point in time?) only about 19% of all their judgments are still "important" as it were, "where importance is defined as a positively trending pattern of citation for a period of at least 15 years."
The life spans of decisions of the various courts of appeal vary quite considerably, ranging from just under six years for the Ontario Court of Appeal to twelve years for the British Columbia Court of Appeal and sixteen years for the Northwest Territories Court of Appeal. What is at work here, it seems, is the volume of cases generated and the rate of change in the jurisdictions legislation. For similar reasons, I suspect, a mere 3% of BCCA, 1.6% of ABCA, and 1.5% of all ONCA cases are currently "important."
CanLII's brief note on the study says, in part, "The study does not propose a firm target, a clear path or a definitive preference in favour of completeness or tailored approaches."
Interestingly, the study has an appendix listing the "Top 100 Cases That Continue to be Cited Over Time," which, as I understand it, is not quite the same things as the hundred most cited cases. Unfortunately the cases are listed using a CanLII identifier (e.g. 1987canlii67) that doesn't tell you which court gave the judgment and that can't be simply plugged into a URL structure to call up the case itself. I'd like to see CanLII convert the list to a set of usable hyperlinks.