The parole board decides release dates on a case by case basis, but there are guidelines. The below websites will give you the crime severity level and parolee risk. You can use those to estimate his time on the Parole Guideline Grid. It depends on whether it is a 1st, 2nd, 3rd or later offense, and many other factors (Education, other arrests, etc.). For VGCSA with intent, he could serve anywhere from 12-26 mos. before being paroled. However, as the parole board will tell you, these are merely guidelines, and he could serve the entire 3 years. Regardless, he is entitled by law for credit for time served in the county jail (O.C.G.A. Section 17-10-11).
I agree with the above answer. Jackson State Prison is probably Jackson Diagnostic, where your nephew will undergo "testing" to determine which prison will ultimately house him. He should expect to be there about six weeks. This means he's got at least six more weeks in custody. After that, it's really anyone's guess. It depends on his age, education level, prior convictions and other things that the Board of Pardons and Paroles may or may not consider in its decision.