Extension of Visitors Visa - Processing Delays
If you think your case is taking too long to get processed, you are not alone. USCIS typically takes lots of time to process any application. There are many reasons for this and beyond discussions here.

Receipt Delay:
When you file your extension, you should receive an official receipt from USCIS within 4 to 6 weeks. (This is just the receipt saying that they received your application and not actual approval/denial letter.)

If it has been six weeks since you sent your application, and you have not heard anything, you can write a courteous, clear, short to the point letter asking for the status.

Sample Letter

While your application is pending, you can track its status online. Online case tracking

Before you can track your application status, you must have received the official receipt and it should have your assigned case number for your application. Depending upon the service center you filed your application, the number prefix would vary.

You can also track your application by calling them, but it may take a lot of time before you can get through, and after too, you may not be able to get any useful information, other than the operator saying your case is pending.
Service Center
Case Number Prefix

Processing Delay:
When you get the receipt that your application has been received, it will mention approximate processing times. If you try to inquire about your case status before that time duration has passed, they will ignore you. If that duration has passed and you have not received any result, you can write a letter to them asking for its status. Again, be nice, courteous and write a short letter. Don't write a big personal story. If you don't hear from USCIS after 1 month, write a similar letter once more. After that you can write another letter every 2 weeks until you get an answer. However, don't write anything that would indicate that you are upset. Never try to threaten or insult them, otherwise you may face criminal charges and your application would be rejected.

Sample Letter