Depending on diet, habitat, and genetics, he could be less than a year, a year, or even older. Males generally grow to be between 6-9 inches, but he shouldn't even reach the lower number of that range by the end of the year if he's not overfed. My RES will be 5 years old this spring, and the last time I measured him, he was just a little over 6 inches.
If you're planning on getting a 90+ gallon tank for him, that would be adequate for him probably for the rest of his life (if you go by the guideline of 10 gallons of tank per inch of shell length and he does grow to be 9 inches).

"You become responsible, forever, for what you have tamed." -Antoine de Saint Exupery-