How often do you update the firmware/ios on switches and firewalls?

I'm a fairly new net engineer so I'm interested to hear how others handle this. Most of our infrastructure was replaced within the past 2 years, so all cisco switches and ASAs are fairly new.

How often do they need to be updated? How do you keep track of new releases and determine whether or not you should update?

For example most of our Cisco 3750 switches are on 12.2(55)SE10, is that current enough? Is it time to update?