Someday I'll find out. In fact, I was considering doing a speed run of White, so maybe I'll see.
Normally though, it's not days - it's weeks or even months. For instance, it took me 300-and-some hours, spaced out over a couple of months, to finish the main storyline in White. That's because my favorite part of a new generation is discovering and getting to know all the new pokemon. I use everything and level them all up at least until their final evolution, just so I can see what they are and what they can learn and do. I spend days on a single route, hunting for good stuff and leveling things up and evolving them and seeing what new moves they learn and such. And I often double back - for instance, I might start to level something up and find out that it learns a bunch of physical moves, but the one I caught is a Modest, so I go back and find another one with a better nature. And so on...
I play for the journey - not for the destination. Though, again, I have been kicking around the idea of doing a speed run, just because it'd be something different for me. We'll see...