Hours per week
Serious play - ~7 hours.
Hittin' 'em around - ~ 10-15 hours.
Focused practice - ~ 45 minutes.
So yeah, I gotta work on that :/
The 'optimum amount' of practice is as much as you can stand before boredom or frustration make the practice unproductive.
A little practice goes a long way when you're at lower skill levels because there's more "ah-ha" moments where you figure out how to execute new things that you couldn't before. But you also haven't hit a million balls yet so you need to spend a lot of time just filling pockets.
At higher skill levels, you already know how to do almost everything, so you're unlikely to have any epiphanies just practicing. You've already sunk a million balls. So practicing is more just to stay in stroke. You need a ton of it to jump to the next level (for example to go from A to A+/shortstop) but if you don't have the burning drive to do that, you can get away with a few minutes a week just "keeping warm".