My parents are on visitor visa. If they stay here for 6 months this year or 3 months in this year and 3 months in next year, then when can they can visit USA again?

As long as your parents have a genuine need to visit and remain visitors, there is no limit on the number of trips they are allowed to make. However, care must be taken to document that frequent visits are not a way of remaining in the U.S. permanently.