USCIS does not detain aliens. That is a function of U.S. Immigration and Customs Enforcement (ICE). A denial by mail is the standard method in which an application for naturalization is denied. Whether the agency decides to initiate removal proceedings against you depends upon a number of factors, including work load, why you are removable, how long you have been a resident and other equities. There is no definitive time frame. It is not uncommon for proceedings to be initiated many years after the denial of an application.
Since you are removable by your own admissions for an aggravated felony you should consult with an immigration attorney who can review your case and advise you of any possible options for relief.