Week one
I've worked with many teams. As a freelancer, the first week is everything. The first week sets the pattern for how I'll contribute.
Day one I read code. Yes, documentation, sure, wiki, the onboarding guide, of course. But, I read the code that runs in production. I trace a request from the entry point to the database and back. I look at what's tested and what isn't. I look at the git log to see who touches what, and how often.
By the end of the second day I want to have submitted at least one serious pull request. It doesn't have to be huge. But it has to show I understand the conventions, the test setup, the deploy pipeline. The PR is a signal: I'm here, I get it, and I'm moving.