Nonimmigrant related Frequently Asked Questions 

I am on H-1B and I want to change my employer. What is the process and how long will it take?

If you are in the United States on an H-1B visa and would like to change employers, your prospective new employer must file a petition on Form I-129 requesting classification and extension of stay for you in the US.

If the new petition is approved, the extension of stay may be granted for the duration of the validity of the new approved petition, subject to the limits on the total length of stay of an H-1B alien.

Under the new law AC21, you can, however, begin your new employment with your new employer once the petition is filed with the INS as long as the employment is under same category and same occupation. It is prudent to wait and start new employment upon receipt of filing of the petition with the INS. (This is the change)(Do not type these comments in parenthesis- 
these are for your information only).

The length of the process would be similar to the length of the H-1B process in general.