The calculation projected actual time to be served is a complex one. It is based on the factors you provide and others. The best way to obtain an estimate is to request one from the inmate's prison counselor and then to verify it with an experienced criminal defense lawyer in the jurisdiction. Other factors that may reduce actual time to be served are "good behavior" and specific institutional policy. An example of a policy reduction is an educational or trade course credit. Also changes in the law of sentencing that may be passed by the state legislature could reduce (but not increase) the time in prison.
Please do NOT use this answer/response to say or do anything regarding your situation. This answer/response is based on the information provided in the question asked and requires a much more complete context than is available in this public forum. BEFORE you say or do anything consult with an experienced Federal and/or state criminal defense attorney in your jurisdiction who will listen to you and your concerns.
Sounds like he got 15 years (3 suspended, 12 to serve) and 10 years post release supervision with 5 reporting. MDOC may let him out sooner, but the Judge ordered him to serve 12 years.
The state of Mississippi does not certify lawyers in any specific category of law and the decision to hire an attorney should be taken only after careful consideration. My profile and any answers to questions given are strictly for the purpose of advertising my services. Nothing said should be inferred to create a lawyer/client relationship or be considered specific legal advice on your situation. Feel free to call me at 662-563-9625.