If you don't consider just shifting or repositioning to be moving around, then I could go a while without moving during a car ride. The longest one I've been on was 9 hours, but that doesn't make a good comparison, considering we stopped multiple times.
I have a hard time staying still for a long time. I can usually stay in one with spot without moving for about 10-25 minutes and then after that I feel like I must move , shift or re position. 30-35 minutes at the most. Part of could be because of my ADHD , I get bored easily with being in one spot and I feel like must constantly be moving. Part of it is also because my butt tends to get tired or hurt after sitting in one position for a long time.
I can if I want to; it just takes determination. I can sit still if I'm concentrating on something, but if someething is out of its place or there's something I really must do, then I'll definitely fiddle and fidget around.
If I'm distracted, I can sit still in a car for hours. Same with at home, barring position changes (at least once an hour if not more). Granted, my answer goes from hours to minutes if we're not counting shifting around a little.
If I'm not distracted, I'll probably be like "why am I stuck here I gotta mooooooove". Also I'll then have to think I have to pee the whole time because that's what I do when I'm stuck on a long car ride. D:
Probably three hours maximum. If I weren't asleep, that is. After about three hours, I need to get up and stretch. Long car rides bore me. Sitting in school too long makes my blood turn into goop. It's boring.
I'm absolutely fine with sticking in one place for a long time. I do it all the time be it sitting on the couch, lying in bed all day (my fave), or being stuck in a car or in class or something. I don't even fidget all that much.
I'm very used to road tripping, as I did it the first seven years of my life nearly non-stop, so I'm well-suited to sit in one position in the same place for several hours at a time without having an issue with it.
I'd definitely start fidgeting. I can't sit still for too long since everything starts to hurt so I have to shuffle around... a lot. xD; This especially holds true in school, since sitting still while listening to a (usually) boring lecture kills me.
Not very long. I start getting fidgety after about half an hour, which makes it hard to sit still in movies. I'm constantly adjusting how I sit just so I can stay comfortable before it starts irritating me again