There was a law at one time that required you to stay for 2 years but of course this opened people up for exploitation by their employers (kind of like a J1 waiver). That was repealed later. Now there is no formal requirement to stay any particular length of time. The issue is that at the moment that you get your green-card, you must have the INTENT to work with this employer PERMANENTLY. There have been precedent cases where the administrative appeals unit and the federal court had to decide these things. The numbers I have quoted are from those decisions.
Now, some smart alecks will tell you: but they have no way to check. Well, when you enter the US on your GC they WILL ask you at times whether you still work for the employer, the other time this can come up is during the citizenship process. If they decide that you didn't actually want to work for the sponsoring company (e.g. by leaving 3 days after the GC) they can retroactively pull you GC and kick you out.
So, this is where the 6 month come from.