Oh yeah, so here's how I actually log these 2-4 hours: If a) I'm only doing X (e.g. not also eating or listening to music), and b) I truly expect not to be distracted (I'm in an isolated location and email and phone are off), then I log that time. Otherwise, it goes into the "everything else" bucket. From my log, it looks like I average only 15 hours/wk under these criteria.
http://www.johndcook.com/blog/2013/02/04/four-hours-of-concentration/
And since this is the Internet, and facts are involved, our gwern turns up there also.