All nonimmigrant petitions and certificates of eligibility indicate the desired starting and expiration dates of the visa. These visas can be approved for maximum periods of three to five years, but you will receive only as much time as you request, and only for the specific period between the dates that are listed on the petition or certificate of eligibility form. When you enter the U.S. with one of these visas, you should also bring the certificate of eligibility or Notice of Approval for the petition. The INS officer who admits you into the country will then know from the petition or certificate of eligibility, not the visa in your passport, how long you are permitted to stay. The officer there will write the date you must leave on the I-94 card.
Permissible activities vary with the type of visa, but all visas function as U.S. entry documents.
The expiration date on your visa does not show how long you can stay in the U.S. once you arrive, but it does indicate how long you have the right to enter the U.S. with visa privileges. Nonimmigrant visas can be issued for any length of time up to a certain maximum allowed by law, depending on the type of visa. Visitor's visas, for example, can last indefinitely. Many other nonimmigrant visas can be issued for up to five years. Citizens of some countries can't get visas issued for the maximum period usually allowed by law.