It depends where you are shooting from, your skills, and your technique.
Below are my expectations for high school teams (boys) I have coached:
- layups unguarded - 100%.
- Within 5 feet of the basket unguarded - 90%
- Free throws 80%
- 3 pointers - 50% for those who should be practicing them
I would expect a good college three point shooter to make 60-70% of his 3s in practice however this differs by kid. I have coached kids that make close to 80% of their threes with no one on them and in games barely get to 30%. And then on the same team there is a kid who is having a great day at practice if he hits 50% of his 3s but in games hits 40%. It is a matter of how well the kid can tune out their opponent, how they handle pressure, and how they maintain and judge spacing.
Also we try to have a guarder on our practice shooters. I ask the kids when not shooting to stand 4-5 feet in front of the shooter. This really helps kids shoot better in games and one of the best routines to get in.
Now the question is - what is your form. There is no use in letting a kid sit there and take 10 seconds to shoot like they are playing horse. The whole point of practicing is letting your body learn good habits. Catching the ball quickly, quickly getting your hands in position and the ball out, 90 degrees, quick back, and then follow through... over and over.
What really separates the good shooters from the people who like to shoot is how focused they are on keeping proper technique the entire time. Over time with close to perfect technique you can get your shot off twice as fast as your average player. Meaning you more or less need half the spacing to get your shot off - which means a higher % in the games compared to unguarded.
The last part seems obvious but I haven't touched on it yet. You ask what a good shooting percentage and I gave you some baselines. But I don't just let the kids sit around and shoot. We run first. A lot. It is really easy to hit 10 shots with a slight fade to the left and back. Really hard to have that fade and be consistent when your legs are tired. On the same thought there are things that each person's body does to compensate for them being tired. Most of practicing is reducing those things. Do your knee buckle in a bit, do your hips turn up to fast, does your feet spray out? Very likely not until you get a little fatigued before shooting.