I found this:
Quote from: http://www.cic.gc.ca/english/resources/manuals/op/op12-eng.pdf - page 29
Specific eligibility criteria
• If the program of study is two years or more, the student would be eligible for a three year work permit.
• If the program of study is less than two years but at least eight months, the student would be eligible for a work permit lasting for a period equal to the duration of their studies. The validity period of the work permit must not be longer than this period. For example, if the student graduated from an eight-month certificate program, they are only eligible for a work permit of eight months' duration.
Exception: Students holding a one-year graduate degree from a provincially accredited post-secondary educational institution in Canada (pursued full-time) after having obtained, within the prior two years, a degree or diploma from an accredited post-secondary educational/training institution will qualify for a three year work permit. This exception also applies to students who have left Canada temporarily between the graduations.
I called CENTENNIAL COLLEGE and they told me that it's posible to apply for a 3 years work permit, If I study 2 post diploma programs one after another.
I will have to call CIC tomorrow, and tell them about this document, just to be 100% sure about what I am going to do
That is from 2008, but Centennial gave me a 2010 version of the document.
This is the link with the 2010 version:
http://www.cic.gc.ca/english/resources/manuals/bulletins/2010/ob194.asp