background check for I-140?

Hi! Did anyone go through the background check during the I-140 application? And how long would it take?

My NIW 140 application has been ted for over a year, and I ted a RFE package in August. After that, there is no response and no online update on the status of my application, so I made phone calls to inquiry. USCIS replied, saying that my case is not out of the processing time, since the "required inverstigation on your background remains open". I have heard that the background check is necessary for I-485, but not never heard it is necessary for I-140 also. if anyone has the similar experience, please share any information you have.

thanks, and bow...