All of the junior devs Ive ever mentored, I would be as detailed as possible with my onboarding, demo the product from end to end, walk them through the codebase, and provide step by step instructions to start up their own dev environment. This usually took a couple of days, depending on the project and the dev.

Then I give them a ticket and say "if you get stuck, let me know"

For the first little while, I am super responsive with them and answer everything immediately and eagerly.

After a while, I start to wait about 5-10 minutes before I respond to questions. Usually they figure out the answers before I even get back to them in these cases, and then I get to praise them for it.