Here's my personal experience with PCC. When I came to Canada last year, I had to furnish PCC's from Canada (where I studied 10 years ago), Singapore, Japan and India. Recently, I applied for PR providing a fresh Japanese and Indian PCC. Reason: When I last applied for a Work Permit to Canada, I was still residing in Japan so if I'd use the old one this time around, I felt it would be rejected as I was still in Japan when it was issued to me.
Similarly, between moving to Canada from Japan, I lived in India for 2 months but I still felt CIC could use that reason to refuse my application and thus supplied a fresh PCC. I could however, not provide a new PCC for Singapore because 1) I never went back to Singapore and 2) Getting a PCC from Singapore requires a formal request from CIC and hence I provided an old PCC along with my PR application.
Needless to say, CIC could still find fault in my application but I totally agree with Asivad that they probably consider PCC's indefinitely valid until you go back to the country where you received it from in the first place.