Some visas are valid for several years, others are valid only for a few months or weeks.
The visa stamp in your passport allows you to enter the United States and may expire after your arrival in the country. See note about Travel to Canada . The Form I-94 should be kept in the student's passport at all times, since it indicates the purpose of admission and the length of time for which his or her stay is authorized. People who need additional time to complete their degree requirements must consult with the Foreign Student Advisor at least 30 days prior to the expiration of their authorized length of stay.