A Post-Graduation Work Permit cannot be valid longer than the length of your studies in Canada.
If the official length of your program of study is:
less than eight months
you are not eligible for this program
less than two years but more than eight months
you may get a work permit for a period no longer than the length of your program of study (for example, if you studied for nine months, a work permit may be issued for a period of nine months)
two years or more
a work permit may be issued for three years