There is no specific rule like that. USCIS hasn't defined a specific cutoff, and it hasn't been tested in court since AC21 was passed into law.
Many lawyers recommend 6 months to be safe -- probably figuring that since it is OK to change jobs 6 months after filing the I-485 while the I-485 is pending, it should be safe to change jobs 6 months after GC approval.
But until there is a court case or specific regulation, nobody will know for sure. So all you can do is decide what you are willing to risk and go for it ... some people feel comfortable at 1 month, some wait 3 months, some wait 6 months or longer. On this forum the only reports of people getting into trouble in the citizenship interview for leaving too soon were those who left in less than a month. But they still got approved, despite being seriously harassed in the interview.