Why does it even matter? Set it to do it overnight or when you aren't likely to be using your computer. It is automatically on a schedule so you do not even have to remember to do it. However, as it happens it is fairly quick, especially once it gets on top of any major fragmentation. My discs have rarely been more than 1 or 2% fragmented whenever I have bothered to check, and they're usually after prolonged periods of me moving files around.