About once every two months.
I only do it a few times a year. I’d do it more often, but for something I find rather puzzling. When I defrag the computer, I must be home, and awake, to move the mouse occasionally. I * have* set the screensaver to “None” but one comes up anyway! A stupid microsoft default one with some walls. None of my settings allows me to turn the sceensaver off entirely, or change “none” to actually not be a screensaver, so far as I can determine from the manuals, so I need to move the mouse ever 59 minutes so the freaking screensaver doesn’t come up and make the defrag program reboot. I’m actually defraging it now, (it isn’t the computer I’m currently using, in case anyone gets confused) because my sims game is FUBAR so I need to do a clean reinstall, and it’s taken 3.5 hours so far…
That’s the main reason I do it.