Jason Kidd took 5 years before the player he would be for the rest of his career emerged. Steve Nash took longer than that. Let me correct myself. It was clear they were both focused on winning and improving their games from day one. Both worked extremely hard in the off-seasons to improve their games. They put in focused, specific practice to improve small aspects of their games. You can see it in the way their FG and FT% improved. They didn't just shoot hundreds of jump shots. They worked on their positioning when they shot, their release, their release moving left, and right, off the dribble etc. Both of them have been held up as examples of off-season workers.
The antithesis might be Vince Carter who, aside from 3P %, saw almost no significant improvement in any aspect of his game from his first year that wasn't directly related to increased minutes. Carter was famous for having a relaxed off-season.
No reason for Derozan not to continue to get better for the next 3-4 years if he puts in the time.