Being a Duke player mostly gives the players a high floor - even recent busts seem capable of having 10 year careers if they want to keep going: See Shelden Williams; Justise Winslow; Marvin Bagley; Austin Rivers. Cam Reddish (ha!).
I don't think it's fair to expect him to be a consistent 35-40% 3pt shooter. He's basically a player that learned how to shoot while in the NBA. I think his perimeter shooting will come and go from season to season, and that's okay. He's not a natural shooter - but he is a scorer and he finds ways to get points. He'll be one of those guys that gets his points in different ways from game to game.
I think a good best case projecting would be a player like Lebron James (ha!).
Career 34.6% from 3. 29% as a rookie to a high 41% midway through his career; 36% this past season. (RJ is currently .357 for his career)
73% from the FT line. Low of 67% in a recent season, peaking at 78%. (RJ is currently .697 for his career)
Where RJ really has significant room for improvement is his 2pt%/finishing - I think this is where he has the opportunity to make great strides. He can do it himself by working on his post up game and finishing better; his coach could do a better job on clearing things out for him in isolation; the team can install a center that can shoot a little for better spacing.
Lebron shot ~48% from 2pt range over his first 3 seasons, then has regularly made ~55% since, and ~60% over the last decade of his career. Lebron is a force of a nature and is a complete freak in that regard. I think RJ more reasonably can target something along the lines of Kobe Bryant or Paul Pierce's career .478 from 2pt range. (RJ is currently .442 for his career)