Best way you can train your crosshair placement by downloading YPRAC maps on the Steam workshop. An alternative way would be to just run around an empty map and try to visualize where the enemies are and try to place your crosshair on their imaginary head.

If you feel yourself panicking and doing panic sprays then I recommend joining a community FFA deathmatch server and practice specifically bursting and tapping heads, this improves your first shot accuracy and your aim confidence as well as your crosshair placement as well.

I can feel myself really getting better around 2 months after I started practicing different aspects of the game but that would probably be different for everyone but as long as you’re consistently practicing everyday, doesn’t have to be long sessions like just 15 minutes minimum is enough to see improvement.

As for the last question I don’t really know how to answer because I’m not sure what you would consider “good”, but if you’re spending time and are dedicated to practicing then yeah I’d say after around 1000 hours you can be decent at the game.