My parents have multiple 5 yrs tourist visa. How long they have to stay in Nepal, before they can come back again in US. Is there any specific time line? 

So far my understanding, there is no hard and fast rule.

multiple bhanyo pachi ta in out garna pai halcha ni as far as my understanding. i dunt think there is a time line. i might be wrong though.

Thank you Sanjali and Kukur, you guys were very helpful. 

They can just leave US and immediately enter and stay 6 months and do same thing again if they want to stay longer in the US. Even they don't have to go to Nepal. Any other country would work. This is what I know.