I believe, the official position is, you are getting a green card on the basis that you agree to work for your sponsoring employer. You are entitled to leave the employer on 'reasonable" grounds. If USCIS is convinced that you were not willing to continue with the employer, even before you got your GC, then they can terminate your GC or Citzenship.
If you happen to leave your employer within 2 yrs of employment, then the burden to prove that you didnt intend to leave before getting GC is on you. After 2yrs, its on USCIS.
Its more a question of intention and situation rather than any time limit. USCIS doesn't "require" any time limit. But, they give you a GC, they believe you have no intention of leaving you employer. But, they do accommodate you if you have to, but only if they feel the reason is justifiable, as they see fit.
Also, IMO, USCIS doesnt give GC, for people to switch jobs. Its only to make it easier for workers to work for their sponsors. Other benefits of GC, are merely circumstantial and should not be abused.