I've had different experiences, ranging from a couple of weeks to a couple of months. Here's what I've noticed makes a big difference:
#1 by far is having an onboarding process. Companies that had a strategy and a method for onboarding employees get people working productively 2-3x times faster.
The other factor I've seen is amount and complexity of domain knowledge. I don't need that much documentation on your code base, I need it on the domain knowledge. I can wrap my head around React components and Ruby POROs, what I struggle with is the jargon and intricacies of your particular industry. Who knew that college housing departments could be so complicated?
Another factor that I've noticed has an effect is my personal familiarity with the tech stack. The more experience I have with the tools and frameworks, the quicker I can hit the ground running. This one is also the easiest to overcome in my experience. The company may have undocumented internal knowledge I can only learn via talking to coworkers, but React tutorials are a dime a dozen.
Lastly, I would say is the complexity of the application. Even in a complex application, you can usually focus on one small part, or find parts that are siloed away and easier to grasp.
The big takeaway I have after thinking about it is that the codebase is the least important factor when it comes to getting acclimated in a new environment. The process of how internal domain knowledge is transferred is much more important.
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