During the visa extension process provided that you filed your application on time, your stay is legal and is extended until the USCIS will make a decision on your case. If your application is approved your stay in the US will be extended from the time your status expired until the time stated at approval notice. For Example, if your B2 status expired on January 1, 2006 and on December 12, 2005 you applied for a B2 extension for additional six months and on March 25, 2006 the application is approved , your will be permitted to stay in the US until June, 30, 2006.