Really depends on both the constituent service personnel and the local USCIS office. For me it took a few days for the senator's office to respond and then like 2 ~ 3 weeks for them to get a response from the local USCIS office.

I sent in an inquiry in a Friday and some called me back Tuesday, today in fact. I was told it could take up to 30 days here in Kansas

I would call not email. I got through to a congressperson same day when I did that. actually their assistant but still there was forms sent etc same day