local interview result

It has been two weeks since our interview. So I went to local office again to inquire about the result. The officer told me that our I 485 cases (my wife and I) were approved three days after our interview and she mailed our approval notice. Could anyone please tell me ususally how long it takes to receive the notice? Do they mail by registered mail or plain mail?

Many many thanks. Special thanks go to Xiaobaitu. You are best.