Legally you are supposed to work for the employer "permanently". Now permanent is a subjective word. Technically it should be forever, but everyone knows that is not possible. The general consensus is atleast 6 months. The whole argument is that if you leave immediately, it may constitute to a fraud as you had argued in your petition for the GC that you will be working for this employer "permanently" (hence the name for GC is "Permanent Resident"). However, bcos of AC21 rulings, I am not sure how much this will hold good.
Again, I am not an attorney, so take my advice as such.