There isn't a minimum amount of time written into law, although the most common opinion given by lawyers seems to be 6 months.
In other cases where people already worked for the sponsor for a long time before GC approval, I figure they would be OK with even 2-3 months. But in your case, it would be more important to do the full 6 months, given that you have never worked for the sponsor.
Below are example questions listed in the adjudicator's manual for them to ask in a naturalization interview if the green card was based on employment. As there is no hard and fast rule as to how long you should work for the sponsor, you have to make your own decision with respect to how comfortable you would be with your employment choices if you had to answer these questions. Of course, if you don't ever plan to apply for citizenship, you have much less to worry about.
– Did you ever work for the petitioner?
– How long did you work for the petitioner?
– In what capacity did you work for the petitioner?
– Why did you leave the position?
– What were you paid?
– Where did you work for the petitioner (location)?
– Where did you go or work after you left your job with the petitioner or instead of working for the petitioner?
– When did you first learn that there would not be a long-term position for you?
