How long should onboarding take?

Great question! Here's some rough numbers from me in a vacuum for an SDE 1:

• After 2-3 weeks, they should have landed a couple commits (~50% proficiency).
• After 3 months, they should be quite comfortable working on the team, writing and shipping high quality code and completing tasks/tickets at a good pace (~80% proficiency).
• After 6 months, they should be completely settled in (~95%+ proficiency).

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:

1. Ship code at a good pace (generally at least a few commits per week)
2. Ship high-quality code (you aren't getting torn apart in code review anymore and you aren't causing production events regularly)
3. Ship code without hand-holding (instead of asking your teammates how code works, you are asking them how to find the best approach)

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.