I'm currently working in a team of roughly 60 developers on a 2-3 million loc system.
First contribution is usually within 2 weeks. We tend to start new developers on straightforward feature work.
We used to start new developers on bugs but found there was far too much tribal knowledge required. We couldn't use bugs to gauge if there was a skills gap.
For senior developers it usually takes around 12 months for them to be performing at a senior level.
More than two weeks suggests the dev environment is suboptimal and requires too much tribal knowledge to get things done. It also suggests the new developer isn't being setup for success by their mentor or the tasks aren't well thought out.
More than four weeks, and you have to look at the new developer and see if they are the right fit. Getting signals from their mentor is really important in gauging this.
Minor bug fixes and tweaks, inside the first week.
Substantial features are usually deployed within the first month. This is limited by our development process.
Of course, there is more learning than contributing on day 1, but even though you don't know anything specific about the system then, you can help with writing or changing when your pair has explained/figured out what it should do.