how ofen to patch in Enterprise Environment?

We're trying to implement a patch schedule in our environment for both linux and windows systems. I'm curious to hear how often other enterprise companies are patching their servers so we can get an idea of what the rest of the world is doing. I've found plenty of documentation from SANS etc on the policy but haven't found much on the frequency of patching. Mainly I'd like some ammunition when the complaints begin from our admins; generally to avoid reinventing the wheel.

thanks!!