AGAIN ... EVERY HUB / Sort is DIFFERENT.. You need to contact your Steward, your BA, go to a Union Meeting and ask, and maybe just maybe READ the Union book and supplement for your LOCAL...
You do not have to work Overtime, you can find another job... haha ok sorry.
I know in my building Part timers get OT after 5 hours. Most everyone stays, but if there is a reason for you to need to leave, work that out with your Supervisor.
I remember reading posts last peak from people saying in their building they were working 8+ hours and told if they left it was job abandonment. Not sure how that worked out, but just what I read.
So the answer is ASK your People with the knowledge in your own Union.