5 minutes ! :) not kidding :)
It'as all about training i guess, i used to work in really quiet places at first, so i got into flow zone real fast, and then i discovered big openspaces so i found a good way to enter it: Music. I just put tha headset on my ears, and in 5 minutes it feels like i'm in another room. As for what distracts me the most : Coworkers ;)