You can do it nearly straight away, and it could become a habit
Falling in with the open-ended nature of the game, you’ll gain access to a lot of different mechanics in Pokemon Scarlet and Violet after the tutorial is finished.
But in the case of sneaking around/stealth, you’ll get it even earlier than that.
Press B to crouch and enter stealth mode
Right around the point where Nemona teaches you how to capture Pokemon (roughly 30 minutes in), crouching and sneaking around will be a useful mechanic.
To do it, just press the B button to enter crouch mode, and it’ll “stick” (no need to hold down the button). To get back up, just press B again.
A lower profile could help you force battles with skittish Pokemon
Just like knocking Pokemon out of trees, you can also use the stealth crouch to your advantage.
Using ZL (left trigger), you can lock on to Pokemon and get a rundown of their level. While locked on, you can also press ZR (right trigger) to toss a ball their way, and initiate a battle.
If you look at the two screenshots in the gallery above, the left image shows off a successful sneak-lock-toss, and I captured the Pokemon shortly after. The second screenshot involves a situation where I didn’t sneak up on the Pokemon in question, and the Diglett dove underneath the ground to escape.
Another actionable bonus of sneaking and tossing Pokeballs on unaware Pokemon: you can catch them off guard, and act first in combat!