ThreatFire enables you to perform a rootkit scan. As you know, there are many other rootkit scanners on the market as well (besides the built in rootkit scan as part of many anti-virus programs).
Performing a rootkit scan once a week seems about right to me. What's your opinion?
Or just use something like Avast which I believe does a quick check at ever startup. Or use AntiVir which can be configured to automatically scan for them when doing on-demand scans.