They are valid for one year. We had an interview today and asked the visa officer and he said one year.
This is what Charlie has said several times.
But for me the issue is, if the Police report is valid when submitted with the PR application, but expires (becomes more than one year old) because CIC takes (say) 17 months to assess the application, does CIC then demand a new police report? My experience says "no", but an acquaintance had to get a new report, and Charlie says yes too.
So, a useful exercise might be for everyone to report:
(1) when they submitted their application,
(2) through which embassy,
(3) when was a final decision given by CIC, and
(4) did they have to provide another police report because their original one had expired during CIC's assessment of the PR application.
This way, if in some cases a police report was required, but in a few cases it was waived (as ours was), then you have ammunition to argue that yours should be waived too, because the practice is inconsistently applied.