I have the exact same issue with police certificates. I called CIC 4 times, 4 different people, 4 different answers and still no idea what I should do. This is the information they sent me:
Police certificates
In general, if you want to immigrate to Canada, you and your family members* must include a police certificate** with your application.
Some countries may ask you to provide fingerprints, photos or a letter from Canadian immigration authorities explaining why you need a police certificate.
For more information on police certificates, please:
Verify the section Police Certificates of the instruction Guide that comes with your application kit, and
Visit our website.
*A family member is a spouse, common-law partner, dependent child or dependent child of a dependent child.
**If you cannot get a police certificate, you must send a written explanation with your application and an original letter from the police authorities confirming that they will not issue a certificate.
And this is what it says on the homepage
http://www.cic.gc.ca/english/information/security/police-cert/intro.asp
No indication of a 1 year or 3 months validity but I remember reading about that on their homepage earlier and call center agents telling me about that as well.
If anyone would be able to help, we'd greatly appreciate that. Thank you