There is no specific rule for how long you should stay in company after getting GC. But some immigration have their own (not BCIS'es) thumb rules - three months, six months, one year etc etc etc. The number varies lawyer to lawyer.
My understanding is that you can change your company very next day after you got GC. But the main factor is that you should be able to explain the reason for your "job change" to BCIS (if they want to know). When a company sponsors you a GC, they expect you stay with them - for long term. So you must show your good faith. And as you know, this "good faith" is very subjective - and depends how you explain it. For example, your company is not doing well (in this economy) and they cannot pay you anymore.
Bottomline, BCIS should never get the feeling that you "WAITED for GC and when you got it you just JUMPED (not from your sofa, but from your company)".
And when you change your job very next day after getting GC, it becomes evident that you were just waiting for your GC.
Normally this kind of things never matters after you get GC. But it matters when you apply for citizenship. While getting citizenship, the factor whether you are a person of good character will be considered. And having "good faith" is part of it.