To my knowledge, the last time anybody challenged the constitutionality of the sentence for first-degree murder was in the 1990s. There were two appellate decisions, one from Alberta and one from Quebec, but that was at a time when the faint hope clause was in place, and both courts found that the actual minimum sentence being imposed was life without parole for at least 15 years, because there was the sort of release valve of faint hope. I don't think anybody has gone back to the courts since faint hope was revoked, and, in particular, there are now provisions for consecutive periods of parole ineligibility that would extend, obviously, well beyond 25 years for multiple first-degree murders. I don't believe there's been a challenge to that yet. I expect there probably will be at some point, but those cases aren't that common.
On February 23rd, 2015. See this statement in context.