I have two questions:1. How long can parents stay in US on visitor visa? As far as I know it is 6 months, is it possible to extend their stay?2. If my parents have stayed in US for 6 months, how long would they have to be outside of US to come back?
They can file for extension while they are staying here. They would have to provide reasons for extensions and then it will be up to USCIS whether to approve it or not
Rule of thumb is that they cannot stay inside US for more than 6 months in a rolling 12 month period. Even then if they follow 6 months in US, 6 months outside US recursively, they may be subject to additional questioning at PoE.