To answer your question it is total combined time over all 21 stages, but it doesn't really matter where he finishes. Let me explain: the Tour is a team competition but for the most part only one rider matters. For team Dimension Data that is probably Roman Kreuziger (17th overall). Basically Ben's job is to make life as easy for Kreuzinger as possible. That means he is going back to the team car to get water bottles, providing the draft when they are not all in the peloton, sacrificing a wheel if their team leader gets a flat, among other duties of a Domestique. Depending on the day he might get a chance to get into a break like on stage 8 i think. Ben is a fairly strong climber, both stages he won in the Vuelta last year were mountain stages, but he doesn't really have the horsepower to compete for the overall classification and it doesn't seem Dimension Data is particularly interested in the King of the Mountains jersey. Therefore, he is basically sacrificed for the good of the team. So other than a stage win, which would be awesome, his placing in the overall doesn't really matter. If he gets a little TV time, and gets their team leader to Paris in a decent position, he will have had a good tour.