How soon do i need to file my H1B extension?

My h1b visa and I-94 is getting expired on 25th sept 2012. My new employer will file my h1b petition(premium) before 45 days of my visa expiry. In case if i will not get h1b approval before existing visa expiry. Is it valid to work with my new employer before getting H1 approval?
How long i can work with my new employer before getting h1b approval?

If filed under Premium Processing you will get to know the decision in 15 calendar days and thus you don’t need to worry about I-94 expiry. You may continue working with for your employer.

If your H1B extension is filed before your I-94 expiry, you can stay as long the petition is pending.