A couple of years ago I was recommending to my customer base that they do either weekly or monthly. However, with the virulence of threats increasing I am firmly giving strong recommendations for the weekly scans for the full disk (+ running processes + memory for rootkits).
Additionally for several years I have been strongly urging daily memory scans (running processes + memory for root kits). Should you find yourself in an outbreak situation like Conficker you will definitely want to know which machines to target with a full scan. This certainly helps. And during an outbreak I set this to hourly. The impact is non-existent without using benchmarking tools and the duration is quick (30-90 seconds).