Depends on the severity of Vunerabilities..
The second Tuesday of the month, MS releases fixes to their OS as well as put out any fixes to things like Media Player or .NET
2 weeks later, if there are is a major fix needed, MS will put out more security fixes.. not just major fixes, but what they have ready.
Also if you have Vista, which has Windows Defender (or XP users that have installed Windows Defender), that gets updated 2 to 3 times per week.. sometimes more through windows update.