Good question. I grew up in a house with no tv and to this day, although we have a physical television in the house, it's not plugged in to anything, so we still don't watch any! We do have dvds, both films and tv programmes, and my son watches tv programmes via YouTube so it's not a total tv black hole.....
But I think really it becomes too much when it starts to affect your behaviour. And that will vary from person to person.
I think in the last year we have greatly cut down on t.v.
I still use my computer for a lot of things e.g. watching maybe something on you tube.
I think children should watch maybe no more than say 2 hours a day.
Adults maybe 3 hours.
There is so much more to life, than sitting watching a box.
My 3 year old has only recently gotten to the stage where she will sit through a whole movie. I'll admit that there are days when she definitely watches what I would consider to be "too much" - mostly when I'm working. I try to stick to no more than one movie per day on a regular day for my 3 year old and my small toddler (18 months) doesn't really watch TV at all...he doesn't have the attention span! But he's happy to sit down and read a book with me.