Tom's Hardware Guide has played close to 500 hours with Windows Vista:
This new operating system is huge: it has more than 37,800 files, taking up a total of 10 GB. Part of this size stems from the fact that the current Beta is for the so-called "Ultimate Edition", which contains all available components, including complete versions of both Tablet PC and Media Center capabilities. In addition, many applications have been compiled in debug mode, so some space savings should occur for final versions once that debug switch is turned off. For our Windows Vista preview, we used Build 5381.