I would speak to your supervisor as they'll have the best and most accurate advice. However, I can tell you my experiences from when I used to be a PhD teaching assistant. Generally, these positions are used more in the arts and humanities than in STEM subjects and while you may have the chance to be a demonstrator or do some teaching, this is unlikely, so I wouldn't bank on gaining too much experience. My own experiences were as an arts and humanities PhD teaching assistant, but I know enough PhD students in various STEM departments to know that this is regularly, but not always the case. In terms of workload, you should be given guidance by the module convener so that preparation doesn't take up too much time, but I will say when you first start teaching that it will take up far more time than you think. Some universities/departments pay a set amount per hour, e.g. £45 (which encompasses preparation, teaching, marking, dealing with emails etc.) whereas others will pay per hour taught and then assign a number of hours for preparation as well and pay for that, e.g. £15 per hour teaching + £15 per hour prep. Generally, it's very rewarding, but can be rather exploitative - you will be paid an amount that doesn't fully reflect how much hard work you put in, so you may spend 2 hours preparing for a lesson because 1 hour isn't enough, but you'll only get paid for 1 hour's prep because the university states that's all you need. I'm happy to answer any questions you may have though.