Six to eight weeks seems to be the average. Have you checked your Documents tab on the website? A copy of the notice should be there? If it is not, you could always use the chat and ask for your interview date that way?
I filed May 18th (under 3-year rule, with pending I-751) and have not heard anything. A friend of mine filed 1.5 months before I did (same FO) and had his interview in early August.
Not me, still waiting. Hoping for an interview in August, friend of mine (not on VJ) filed three weeks before I did and he got his interview scheduled in August