How long of a gap is needed before my parents can visit me back in the US?

Hello forum,

My parents are here finishing up their 6 month stay in the US soon. They will be heading back to India for a few months but plan to return here for another 6 month stay. How much of a gap are they supposed to give before they are back here in US? Is it a defined gap, like 6 months etc. or can they return even after a couple of months or so? Please help. Thank you!

I personnally recommend gap of 6 months , though there are instances of people going even with less gaps