If Time Machine works like Snapshot technology on NetApp Filers, then it will have a smallish overhead on your HD, as the previous versions of the file will not all be saved, just the changed disk blocks. On NetApp you can configure it depending on how far back you want to go, but typically people dedicate about 15% of their hard disk space to it. Remember though that on a NetApp every piece of info is changable user data. The OS is normally on a different volume, so the percentage on OSX will be less than this.