Once you've installed Apple's Security Update 2011-003, your Mac should check for new malware definitions every 24 hours.
But if you want to know precisely when Apple last updated the definitions on your Mac, Hints reader joelbruner has a quick way to check.
Launch Terminal, and paste in this command:
defaults read /System/Library/CoreServices/CoreTypes.bundle/Contents/Resources/XProtect.meta LastModification
Terminal will spit out a timestamp, which marks when Apple last updated the data that your file contains. (Note: the timestamp does not represent the last time your Mac checked for an update.) If you're concerned that your Mac's malware definitions may be out of date, you can run another Terminal command:
Terminal will prompt you for your administrator password and then check with Apple's servers for an update. To confirm that your definitions are now more current, just run the
defaults read command above again. Remember, though, that just because your Mac can check for new malware definitions each day, Apple won't necessarily publish new definitions each day.