It used to be 300 an hour when I was a loader but that may have changed and speed comes with time. As long as you are showing improvement from day to day, they should keep you unless you have attendance or punctuality issues. After you make seniority, they are pretty much stuck with you so go at a pace that is safe.
300 is definitely doable and so is 600. I topped out at 1,000 an hour one peak when I was racing another loader. My gut didn’t stick out as far back then.