As I'm starting to join a team soon(and knowing this answer varies by team, level, and company), I'm wondering how long does onboarding take and what does it mean to be fully onboarded within your team
Great question! Here's some rough numbers from me in a vacuum for an SDE 1:
To caveat all this, the real answer to this question, like a lot of questions, is talk to your manager. To learn more about that, I highly recommend this Q&A from another onboarding engineer around how long their first JIRA ticket should take.
what does it mean to be fully onboarded within your team...
For an SDE 1, your performance is almost entirely dictated around your technical proficiency. This means that you are very fluent with the code and are able to do the following:
On top of the above 3, I would expect a solid SDE 1 to also review some code (especially those of other SDE 1s and interns) and have some decent relationships built up, either with other engineers and/or with the designer/PM (if you work on front-end).
The quality bit is what I see a lot of junior engineers get tripped up on, so check out this Q&A to learn how to write actually good code.
Lastly, I recommend this Q&A around how to write great code faster.