Right after I UN-install anything or If it has been longer than 3 days I run it anway, you wouldn't believe how fast the .log files pile up
Like Osaban, I use CCleaner daily, but I run it prior to shutting down in the evening. I use CCEnhancer too. And I also switch off from time to time with BleachBit, or I use them both in tandem.
Same for me, it is a good prevention against malware, which could run from temporary internet files to gain elevated privileges after startup.
Daily, sometimes multiple times since Chrome is a bit bad about deleting browser data through the normal means.
I voted monthly as there was no option for anything less than that. I only run it a few times a year. I see no reason to use if more often than that. I don't want to clean usage records ever, and Window's own disk cleanup does a reasonable job under Windows 7, so it's not really of much use to me. Also I find it slightly annoying that when I first install it, I have to carefully uncheck anything realted to usage data. I want to clean junk files and browser caches, but nothing else, not even cookies. There are some instances where cleaning usage data can speed up a computer, but for the most part I'd rather put up with a slower computer than clean this.
Not really these days. But I keep my toe in so I don't go totally brain dead from thinking about OSs only 1 way.