A true gentleman keeps on top of his computer, and scans it once a week at the minimum. That's all it needs. However, I don't even really need to because if their is a problem the tech's at the building we call school just re-image it.
Very rarely. Last time I did it, it was after a year and it showed nothing.
Avast has a nifty extension for all commonly used browsers which blocks malicious sites. Very helpful when browsing through torrent sites.
Then I have Malwarebytes, but I seldom use it, and it's not really useful because the free version doesn't run in the background and thus doesn't intercept malware.
Keep note that antivirus programs tend to be overzealous and show lots of false positives. This happens a lot with NoCD cracks and certain utilities which make use of suspicious, albeit harmless, code. Usually, an infected executable is pretty obvious to spot.
I usually scan my computer at least once a month, or when it crosses my mind. I've never really had a virus or a worm or anything of that nature (besides a heuristic worm-virus-thing I managed to destroy before it could damage my computer), so I'm not that concerned about scanning. But yeah, for protective reasons, I try to do it at least once a month.