Visas for business or vacation trips to the United States typically allow up to a six-month stay. You must show that you have a home outside the United States and money to return there.
You may apply for an extension of your visa, for up to a total stay of one year. You must apply before your current visa expires.
A student visa allows you to remain in the United States until you complete your educational program, regardless of how long that takes. You must meet several requirements, including full-time enrollment in school and only accepting certain types of employment.
Citizens from certain countries have specific durations of stay in the United States. For example, as of 2010, Iranian citizens may remain in the United States for only three months.
A border agent will put a Form I-94 into your passport when you enter the United States. If it specifies a date, you must leave the country or receive an extension by that date, even if it is before the typical duration of your visa type would end.
