As long as you have received a receipt number while they are processing your case and they have not send you denial notice asking you to leave within 15 days,(Certain date if denied), then you are fine and legal till then. If they deny your application for extending visa they will send you a notification saying please leave within so and so time frame.................. on the denial letter. You are legal until you receive a response from INS.

Keep the correspondence  letter safe with you because if you are to travel back to USA after some time again , they may ask for the time you overstayed and that letter proves that you were waiting for you application to be approved. They asked for my parents and luckily they had saved the documents AND IT HELPED.

I hope this helps.

Regards,

Haribansa