I mom  B2 visa expired on 10 March. I applied for extension but it is  denied on 18 March. I am just wondering when does she need to leav the US. I hardly find answer. Please help me. Thanks

 I believe she will have to leave the country asap.   She might already be out-of-status.  That is why they recommend that you file for extension at least 45 days in advance to the visa expiration date so that in case of denial, the person is still within status.  Sorry bro.

Thank You Riten bro
But they gave me denial decision after the day of I 94 expired.
That is my mom i 94 expired on 10 March but they send me denial letter only on March 18.