Manually? Maybe once every three or four months. I have software that scans my laptop constantly for these things, and individually scanning my files takes the better part of 4-5 hours, and I can't do anything else in the meantime, so I rarely do it. I need my laptop during the day, and I don't want to leave it running overnight.
I figure if there is anything I need to know about, the monthly report I get will tell me, or I'll get a pop-up in the corner of my screen telling me not to download what I'm trying to download, or view the site in question. It works well enough, and since I pay monthly insurance on my laptop (which is a lifesaver, because I accidentally broke the screen once and it would have cost a fortune to replace it) I'm not TOO concerned about it. At least, not too much...