Interesting question. I think I'd naturally space out a significant portion of the day, but I kind of intentionally keep myself from doing so. Thing is, it's totally possible to "space out" while appearing present. For instance, I like to play minesweeper and solve rubik's cubes (the 5x5x5's are great for killing time!). While doing so, my right brain is in la-la-land, while my left brain is doing the rote problem I've given it.
The desire to space out is always in the back of my mind, though. So, I didn't vote in your poll, since my answer is different from any of those, but my answer is really "as much as I let myself". Some days, it's more than others.