While I was waiting for secondary in ATL, I noticed this case. An elderly lady was wheeled in and the officer asked how often she was in the US. Twice in the last year was the answer.
The officer then warned her, you have been here 12 out of the past 14 months. If you ever visit again, we will cancel your visa and send you back! If you want to stay here, ask your children to petition to live in the US.
6 months out of a year seems to be about right. Anything more than that seems to raise their suspicion of possible migrant.