When a company sponsors you for an Employment Based green card, it is because you intend on being employed with them on a permanent basis.
There are two key word in statement I made. Intent and permanent. What is permanent? Certainly not the rest of your life. Some lawyers advice that 6-12 months (rarely more) is sufficient. That's where the 6 month rule of thumb comes into play. The second part is intent. Can you prove that you intended to stay with your employer permanent when the I485 was filed an while your were waiting for an approval? Again, some would say that staying 6 month would prove yes on both counts.
There's nothing set in stone, although leaving your employer 2 days after getting your green card might raise some eyebrows.
rgds,
sadiq