I think this depends on a lot of factors. For a developer, these are a few factors I can think of:
The third point is quite important for products that are quite complex, and can really slow you down if you have to figure out most of the context yourself as opposed to being given guidance on where to start etc.
I find good tooling can help eliminate a lot of mistakes and give you more mental capacity to focus on learning about the codebase, rather than having to worry about potentially shipping code with your changes that might break things.
To answer the main question, from personal experience, I'd say somewhere between a month to six months 😅
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