How much work and how involved you are depends very much on you. Most TA's look at Lab work and grading as a chore that they had to do to as a means of paying the tuition and fees. In other words a burden. However, what many do not realize is that this experience can be as rewarding to you as to the students when you do it well. You in fact can learn several skills at the same time the students are learning theirs. You learn how to speak and explain physics in front of a group of people, you get to relearn all those undergraduate principles and to look at them from the perspective of a student who are learning this for the very first time.
How much effort you put in depends on you, and can also depend on how the course is structured, as well as how much of a freedom the class instructor gives you. When I was doing it for the first time, I was basically trying to feel my way around, getting used to the equipment, doing the experiment myself before hand, and essentially trying to anticipate where the students might have some confusion or issues with either the lab instructions, or the experiment itself. I stuck rather closely to the experimental instructions at first time. During the lab itself, I seldom just sat down and let them do the experiment. I tend to walk around, looking at what they do, and some time ask them what did they observe, or why do they think they were asked to do something a certain way. I try to make them pay attention not only to the physics principle that the experiment is trying to illustrate, but also the experimental technique and observations that are relevant to doing a good experiment.
After I've done it for one semester, I managed to be a bit more "creative" with the experiments in the next semester. I altered minor parts of the laboratory for almost every single experiment and made it slightly different. In most cases, it was more of a time crunch, since I noticed that the students simply didn't have the time to finish everything, and I'd rather they learn a few things well, rather than do a bunch of things but got very little out of it. However, another motivation for changing the experiment slightly was to prevent cheating. Many students, it turned out, simply copied the lab report from previous students who had taken the same course. When I changed it a little bit, it made it a bit different and so, they can't just copy it off. Still, there were still students who handed in reports containing parts that were not set up for, or were not done, or were done differently (let's just say that they had a lot of explaining to do, and the whole class now realized that I meant business. There were zero copying after that week.)
All this will mean a bit more work on your part. It certainly did for me. However, I see 2 reasons for doing this:
(i) I want to be fair to students who actually ARE doing the lab report themselves. If you do not weed out people who simply copied off "perfect" lab reports, then these people are getting good lab grades while those who actually did the work aren't. There is then very little incentive for people to play it straight.
(ii) there is a bit of "dignity" involved here too on my part. I simply cannot overlook someone trying to get the better of me by cheating. I think this is more of a question of self-respect, that you won't let someone take advantage of you.
TA'ing lab session can be a highly rewarding experience. You get to meet and deal with students, who can range from some of the most intelligent and eager people you'll ever met, to the laziest bum with the can't-be-bothered attitude. You should consider it as a challenge, and a practice for your career as a physicist, because these are the range of people who will encounter, and some of them could be from your funding agency! As long as you consider this as being your learning experience as well, you'll never look at it as a burden that you have to do.
Zz.