This article is from the FAQ, by with numerous contributions by others.
There is NO prescribed time limit. There are a couple of considerations
that must be borne in mind. The basis for getting a GC are that you took
up a "permanent" position. If you leave too soon, INS may claim that you
did not intend to take the job up on a "permanent" basis.
"Permanent" does not mean forever. But it also does not mean that you may
leave the day after you get the green card. Unfortunately, there are no
bright line tests in this area. (Answered 06/24/94 -- Please leave the
date in, so people know how recent the answer is.) It is imperative that
you seek the opinion of competent counsel in this regards.