It would depend on so many factors. My children are both at university overseas and they go backwards and forwards every few months (spending approx 7 months outside the US each year), they have both been taken in to secondary when entering. But as soon as they can prove they're only outside the US for educational purposes and that their only home is in the US, their parents are here, their jobs are here, they pay taxes here, etc, they are waved through instantly. There's no set '6 months or more' that will flag, just a regular pattern of travel can do it too.
If you're questioned, it sounds like you don't have a home in the US anymore? So what other ties do you have to show you are still LPR's other than occasional work responsibilities?