Not long, there's no lag whatsoever. A few hybrids already do that. My friend's Honda Insight does, pretty freaky, DEAD SILENCE. I always say "dude- your car died"
The article is probably wrong tho- on autos it should restart as soon as you get off the brake, and it's up and running before you can get on the gas. For manuals it starts as soon as you touch the clutch, before you even press it down. And the starter is more sophisticated- there's no cranking up the engine.
On a similar note, I was told BMW is coming out with a V-10 that switches to 5 cylinders to save gas?