I've never seen anybody ask how much time is too much time spent reading, drawing, putting together puzzles, or playing board games. Yet as I see it, reading on a tablet is just like reading a book whose pages can never get ripped; drawing on a tablet is like fingerpainting that my daughter can do no matter what she is wearing; playing puzzles on a tablet is like putting together puzzles without the ability to lose pieces or mix them up with those of other puzzles; playing a game on a tablet is just like playing a real game but without having to worry about remembering the rules or keeping the pieces out of the baby's reach.
As for television, I'm OK with reasonably educational shows. If your child has to sit in a chair listening to talking for 5 hours a day, does it matter if it's in a school listening to a teacher versus in your living room listening to the TV? Of course "school" for toddlers
is generally a lot more engaging than the lecture-style classes of older kids, but it's generally not as educational either.
I prefer human interaction to watching TV, but there's no way I can compete educationally. For example, I can tell my daughter "Remember, look both ways before crossing the street!", but on TV they can write a catchy song, choreograph a dance, make a skit with props and dialog, and then repeat it 10 times a day all month long.
I don't like plopping my kids in front of the TV all day, but other people in their lives do and honestly, I can't say that it seems detrimental to their mental development. When my 2-year-old starts leading an imaginary marching band, talking like a pirate, holding a toilet paper tube up to her eye and calling it a "spyglass", or saying something in Spanish, I know it's because she learned it on TV. There's just no way I would even think to teach her all that stuff.