Rajiv talks about this topic here.
http://immigrationportal.com/showthread.php?t=154533
My own understanding is much the same as Rajiv's take. It seem the majority say six months, but some say 3 months and others say one year.
No, there is no clearly written rule about how long you must stay. It's all about "intent," which is wide open to THEIR interpretation, not ours, essentially.
Speaking of AC21, that is about BEFORE GC, and this question is about AFTER GC, that's the difference. Regarding AC21, the principle of employment-based GC, i.e., to fill skilled labor which cannot be filled by citizens/GC holders, is still substantively maintained by the rule that your new job position must be the same as or similar to the previous one. This is not the case with job change after GC. So in my opinion, there may be some relaxation in interpretation, but one should not draw a firm conclusion about immediate job change after GC based on the fact AC21 allows for job changes.
In actuality, it is rarely if ever asked at your citizenship interview how soon after GC you left your sponsor's job. So you would be mostly fine. That does not mean the risk is 100% gone, though (I know one case (a friend's former colleague) who got into trouble because of this). You cannot completely rule out the possibility that your former employer complain to CIS about you leaving right away, for example.
So, I cannot say you are 1000% without any problem whatsoever, as long as there is a risk possibility. But the actual risk is most certainly very low. The rest is up to each people, i.e., whether or not one is willing to take this low risk, or whether or not one cannot take the "what if?" feeling. It's all up to you.
