Form I-797 will issued to the prospective U.S. employer by the Department of Homeland Security (DHS) upon completion of Form Form I-129 and its approval by U.S. Citizenship and Immigration Services (USCIS). A copy is usually sent to the applicant and to the registered legal representative. The original full page of Form I-797 must be included with certain visa applications.
If this is the initial application for status in a labor visa category, the application for a visa may be made up to 60 days in advance of the date on the approved I-797, but initial entry into the U.S. may not be made more than ten days prior to the validity date on the I-797 or after the validity date has passed.
This is not the case for persons extending or revalidating their existing visa status. Such persons may apply for and be issued a visa at any time with their currently valid and new I-797.
The I-797 is issued by Citizenship and Immigration Services (CIS) upon their approval of the Form I-129 filed by the employer.
You are permitted to stay in the United States for the duration of your I-94 (the card you were given at the time of your initial entry into the U.S.) The validity generally matches the time indicated on the I-797 (Notice of Approval.) Even when your visa expires it is the expiration date on the I-94/I-797 that counts. However, if you plan to travel outside of the US/Canada/Mexico, you will need to get new visa to reenter the U.S.
