When you enter the US, the USCIS office at your port of entry will issue you a Form I-94, in your passport. The USCIS inspector will write the duration of your valid status in the US on your I-94. The date on the Form I-94 is the date to which you can stay in US. That is, if you have "duration of status" you may remain in the US as long as you are in the same job or same category of job. It is always advisable to apply for your extension of stay with INS a few months before your I-94 expiration date.
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