An H1B is usually valid for a period of 3 years at a time, extendable by another 3 years on renewal.

Your legal status depends on the expiration date written on the I-94 card by the INS officer at the port-of-entry. As long as the mentioned date is valid, irrespective of whether your visa stamp in the passport has expired or not, and you are continuing to work for the sponsoring employer under the terms of the petition, then you are legally permitted to be in the US. However, you would need to renew your visa stamp if you plan on leaving the country for a while.

For the purpose of renewal / extension of your H1B, the following documents need to be submitted to the US State Department:

• Valid passport (at least six months validity remaining) for the applicant and each family member.
• The original of the form I-94 issued to the applicant by the Immigration and Naturalization Service.
• Form I-797: The latest INS approval notice. This has information on the applicant's current employer and indicates the approval dates or renewal / extension dates.
• Employer's Letter of Support addressed to the Visa Office of the Department of State. This letter should contain information relating to the applicant's position, the nature of the business or profession, and an     explanation as to why the visa is needed.
• A money order or certified check, payable to the Department of State, for visa fees.
• A properly executed visa application form for each person requesting renewal of visa.
• A self-addressed postage-paid envelope for returning the documents.