This is false. There's a lot of misinformation on the internet about this. Speak to an immigration attorney and you will discover there's no mandatory period whatsoever.
See also: Rights and Responsibilities of a Green Card Holder (Permanent Resident)
http://www.uscis.gov/portal/site/uscis/menuitem.eb1d4c2a3e5b...
", the approval of a Form I-140 employment-based (EB) immigrant petition shall remain valid when an alien changes jobs or employers, if: • A Form I-485, Application to Adjust Status, on the basis of the EB immigrant petition has been filed and remained un-adjudicated for 180 days or more; and • The new job is in the same or similar occupational classification as the job for which the petition was filed. "
[1]Policy and Procedural Memoranda on the American Competitiveness in the Twenty-First Century Act of 2000 (AC21)
http://www.uscis.gov/USCIS/Laws/Memoranda/Static_Files_Memor...