Hi Ashu,
You are right that it is valid for 1 year but not in all the cases. Let's say I was in Australia from 2007 to 2011. If I never visited the country after my PCC then it can be considered as final one. No need to get it again.
Also, as per my knowledge, PCC is valid till 1 year and you need to get a new one but till the time they issue you a PR. Not like you enter Canada. Because they have a strict expiry date of the PR visa before that date you need to arrive in Canada, PCC makes no sense after PR.
Thanks,
Parth