Once a week sounds about right if you are actively online.
If not, 2-3 times a month should be ok. It just depends on how much time you spend online surfing.
This is pretty much a personal preference type of question. You have to scan your computer but how often is up to you. You have to judge what is a happy medium that you can live with. You are going to get a lot of varied opinions, but that is what they are - opinions. Everybody is entitled to theirs but you be the judge as to how often you scan.
If you have good defences (eg AV, Firewall, Spywareblaster, Firefox, Spybot Realtime) you should rarely be troubled. I usually scan about once every 2-3months and cant remember the last time I had something nasty.