Hard drive slack space wiping questions

Hey men, I am using a pretty nice hard disk/windows cleaning tool, which I have set to run on schedule each morning at 6am. The daily cleaning includes all the basic cleanings, such as windows temp files, browser files, recycle bin, and etc. There are a few options in the cleaning utility that if used, make the cleaning process take too long, so I dont include them in the daily cleaning, these are wipe file slack, wipe unused space, and wipe slack of existing files. I was thinking maybe to create a new scheduled task which would run monthly, to do these tasks, and omit from it the tasks performed in the daily cleaning. I figure if i schedule it to run early in the morning, it may be done by the time I get home from work, but I dont know, honestly. I'd hate to come home and find it just 12% done on my 2nd partition, with 4 more to go, and just have to click it off. So what are the opinions of the users as regards this matter? I'd like to know what you guys think of the time it would take for these task on roughly a half terabyte system, as well as importance of doing it to begin with. It may not be practical even on a monthly basis?