Upvote:2
There is no published requirement. The presumption under U.S. law is that every visitor visa applicant is an intending immigrant until they demonstrate otherwise. Therefore, applicants for visitor visas must overcome this presumption by demonstrating:
• That the purpose of their trip is to enter the United States temporarily for business or pleasure;
• That they plan to remain for a specific, limited period;
• Evidence of funds to cover expenses in the United States;
• That they have a residence outside the United States as well as other binding ties that will ensure their departure from the United States at the end of the visit.
https://travel.state.gov/content/travel/en/us-visas/tourism-visit/visitor.html