The USCIS will send you a receipt with a 13 digit case number. “The approximate processing time will be indicated on the receipt.” You may be given a biometrics appointment so you and anyone else on the application can be fingerprinted and photographed
If you file your extension request on time, you may be permitted to stay in the US for up to an additional 240 days depending on the reasons behind your request and other factors.
You can check the status of your request online through the receipt case number.
If the visa extension is approved, you will be given a replacement I-94 with the new expiration/departure date. You should make copies of this I-94 so you’ll have them for subsequent trips to the United States.
If the visa extension request is denied, you’ll receive written notice of the denial. You will need to leave the US immediately.