Essentially, you're validating US embassy's point your parents have immigrant intent.
The more they live in the US the better. There is no black and white rule. At any point, CBP may presume they're not residing in the US. If they only spend 4-8 weeks a year out of the US, it would be much harder to accuse them of not living in the US. The closer you get to 6 months a year out of the US, the more probable it becomes CBP asking them about their travel and life in the US.