I don't bother flooring my accelerator, as doing so just causes the transmission to pop into second which is terrible at accelerating from a dead stop, not good for the engine, and embarrassing when the engine whines like mad and you barely accelerate. If I were to guess, I would estimate I put the accelerator in 1/5th of the way, and add more as needed.
I heard an American auto exec on TV once state that Japanese auto-makers put a lot of effort into improving their acceleration "off-the-line" with their automatic transmissions (I drive a Suzuki Vitara). I've beaten many a (regular, not sport) car "off-the-line" until I hit 2nd gear at which point they can pull ahead until I get into third before them. Of course any kind of acceleration beyond third gear is just wishful thinking at that point. I think some people are just lazy with their shifting. It doesn't take my auto more than a few tenths of a second to switch gears, though. I've also been driving this car for almost 6 years now, and know how to work the accelerator to get it to shift and accelerate how I want.