Ideally one should get 1 year PGWP after 1 year of actual study. When someone mentioned he/she knows some people got more than 1 year's PGWP after just 1 year of studies, I totally agree as I know such cases too! So what happened is, when these guys graduated, their certificates had incorrect duration which was 2 years and actually the program was just for 1 year. But then they got just lucky.
Now, 2 years of program fetches three years of PGWP. However, what really happens on ground zero is, this is something people have tried and got the desired result ; so student completes 1 year program and enrolls in another program of just 1 semester then applies for PGWP and gets three years PGWP (it worked for many people, but I would say it's like trying your luck).
@Krish,
If the certificate states duration as 2 years, you should be good to go with 3 years PGWP. So it basically depends on what college states on your documents.