Twice everyday for me with just good ole' sbybot and ad-aware.
Once a week for me, where I run a full scan with AVG/Ewido/SS&D/Ad-Aware(never find anything though).
I run Ccleaner several times a day, only takes a second or so. It keeps my system running smoothly and gets rid of useless temp files etc.
Why do you run SS&D and Ad-Aware twice a day? I would`ve thought it was a little unecessary to run them so frequently.
P.s Check your pm`s.
My AV has real-time virus and spyware scanning on (has never found anything (is that good or bad? )). I do not use IE and I do not download/run/install crap programs. Removed all IE icons too to make sure other users don't get any ideas.
SS&D gets run maybe once a month when I'm really bored (only finds cookies).
I only ran it once when i first installed it. since then, it just runs in the background. i rarely do Spybot or Adware scans, only when i remember to, and feel bothered. but i keep the definitions uptodate.
Personally, I have AVG set to scan each day at 8am, and if I'm awake and using the computer at that time it gets switched off. I check the test logs once in a blue moon because I'm confident that I woun't get anything, and my confidence is justified by my track record. If I did get something I'd know - either my the misbehaviour of Windows, or from my firewall...
...and as Nodsu said, I have real time scanning anyway, and can scan downloaded files etc from the context menu if I need to.