H1B/H4/H4 EAD extension

Our visa is expiring Aug 2017, if the employer files the extension under normal processing to get the extension H1B/H4/H4 EAD, how much time will it take for the extension process as my wife is also working on H4 EAD. Just don’t want my wife to get into any job interruption as she wont be able to work incase NEW EAD card is not issued before Aug 2017.

The processing center will be USCIS - Vermont Service Center.

Would Appreciate your best advice!