I'd say it is really child-specific. Some kids will watch a little and then move on to something else. Some kids would happily watch all day to the point that they forget to eat, potty, etc. I have a child who admittedly watches more than the recommended amount now at 4. I police content heavily. There are some shows he can watch and be fine (NOVA, Cosmos, some PBS kids shows, tree fu tom). There are other shows, where even watching a little causes significant trouble for him self regulating. If he watches anything on Disney Jr or Nick Jr, he literally has difficulty behaving for 1-2 days, and it has consistently been this way for years. Other kids can watch the same show with no visible change. So just keep an eye on what your specific child is doing and how he reacts.
If possible, use Netflix or dvds so you can skip commercials. They are just awful.
Also, balance... We read for hours each day and my son is practically constantly in motion. After a 4 hour hike, if he asks for some shows, I don't really worry about it too much anymore. I was stricter when he was younger.
I am very strict on content though. I don't allow shows with behavior that wouldn't be acceptable in my home. In shows with villains, they must be pretty mild (Wild Kratts ok, but no shows with truly bratty kids and no violence for us.). For what it's worth, I also heavily police book content at this age.
