The onboarding link will expire on your start date (at 00:00) so make sure you complete all tasks before that date.
Before your start date, you can click on the link and access onboarding as many times as you want within the same session. If you close your session then you will be informed that the link has expired and will be prompted to provide your email address in order to access onboarding again and continue from where you left off.
Once you add your email address and you click the button, you will receive an email with a new link.
Note: This link will expire in 3 hours and can only be used once.