- If you choose to leave a lab:

So long as the lab isn't full (less than 40 people) and is still set to public (it hasn't been set to invitation-only) you can return to join the lab in the future. You may search for it by name or ID and then tap Join to rejoin the lab!

- If you've been expelled from a lab by the lab's captain:

The captain may decide the duration of expulsion (1 day, 1 week, 1 month, or unrestricted (return at any time)). You must then wait for this time to elapse and ensure the lab is both not full and still set to public to be able to once again search for the lab and rejoin it.