That can vary quite widely, and depends to an extent upon how competitive you are as a candidate on your particular nomination slates and your particular file. For example, if you are a very competitive candidate and are deemed desirable to have as a midshipman, but have somehow managed not to have taken chemistry or some other course of study deemed vital to your success at USNA, they may offer a prep school early. Normally there is not a lot of prep school offers made until later in the cycle; eg, after April 15 or therebouts.