Let's hit this with some math. Let's call "several hundred" 300. With a shagger, figure he can get off about 1 shot every 4 seconds, or about 15 a minute. For 300, that's about 20 minutes of practice.
I hope that he shoots about 1,500 shots a day, minimum, which would be about 2 hours of work on his shot. Him figuring out how to hit a 3 at 35% is the difference between the NBA and the EU. If I was him, and I had NBA aspirations, I would be on the court shooting until my arms fell off, which realistically is about 6-8 hours per day, in 2 or 3 sessions, for about 4,000+ shots a day, every day, no exceptions.