As soon as you can solve an issue! I found a bug in a Yelp WordPress plugin back when I barely knew any PHP and figured out how to fix it, because I needed that plugin to get paid for the project I was working on. I submitted a pull request and they accepted it! That's the first contribution I ever made to a corporate project. The people in charge of the repository will give you feedback if they want things changed, and can provide a lot of guidance. Just start committing and you'll learn a lot.