There is no specific length requirement by the immigration law

I am assuming you obtained GC through employment-based sponsorship.

If that is the case, you must show that you have a good faith intent to work for the sponsoring employer on the "permanent" basis if questioned by USCIS.

The reality is that, as long as you do not piss of your employer, you can change the job asap.

The rule of thumb is 6 months.