The duration of stay depends on the specific visa program. For nonimmigrant visas, such as tourist or student visas, the authorized period is typically mentioned on the visa or given on arrival. Work visas may be granted for a specific employment period. Immigrant visas, such as family-based or employment-based visas, generally grant permanent residence.
The U.S. Bureau of Consular Affairs has a list of standard timeframes allowed by category of visa. Ultimately, it will be up to the immigration officer at the port of entry to approve entry and set the permitted duration of the traveler’s stay in the U.S.