IMO it's not a switch that gets flipped and suddenly you're productive. It's more like you slowly get more accustomed to a codebase (assuming it's not all greenfield), and it's highly dependent on the complexity of the system. In my experience, boring business automation: 3~ months; Quantum computer control system: no. Jokes aside, it took about 6 months for me to feel comfortable with the super complex, extremely specialized system that I've been working on.
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