You won't be holding the same retractor in the same place for 8 hours, but you may go over one hour for sure. You will be holding it for a time, then the surgeon will move your instrument a few inches to the right and you hold it again for a time, then he will swap your retractor for a longer one and you hold that one, and so on. The hardest parts tend not to be the holding as much as the contorted position you may have to be in to give the surgeon room to work without your shoulder obstructing his arm movements. So you may be leaning in from the side, and not really able to see where the toe of the retractor is. And folks keep pushing against the retractor with the instruments they are using, causing it to gradually slip out and then you get yelled at for not holding it still (even though you can't really see that it moved because you are off to the side. You will have a number of surgeries where you just watch, but they tend to be the ones that you are watching because there is no room for you to be at the table, and so as a consequence you can't really see. And watching without seeing is a bit frustrating, but you still have to seem interested if you want to make a good impression. So your goal is generally to be involved in the surgery because then you can sometimes actually see, and then often get to do other stuff if you do a good job with the retractor (cutting sutures, throwing stitches, suctioning, irrigating, using the electrocautery... the fun never stops

).
In my experience surgery is harder on the feet and the bladder than the arms. Get good shoes and never pass a bathroom.