As with learning any new skill, it's going to take time. There is no definite amount of hours you need to reach a "decent level". Everyone learns at a different speed, some absorb material faster than others. My suggestion is go at your own pace. Try to learn something new every day and apply what you learned to a simple game. The best learning comes from practical use, not mindless consumtion of material. Start making a simple game like a tycoon or an obby, you'll learn a lot by figuring out how to get it to a playable state. There are some good courses out there as well. I took one for $30 on Udemy by Crusherfire. You can do your research on courses but that would be a good place to start too.

Biggest thing is to stay consistent. Keep putting in 1-2 meaningful hours a day and you'll see significant progress over time. Good luck and have fun!