In my experience it gets updated between 1-2 times a month. However, some people os reload their machine or want configuration changes to their kernel. There was a month where a new kernel came out every week mainly for the transition from 2.6.18 to 2.6.19
It really depends on your update schedule though some people only want security updates for minimum downtime due to restarts and those are few and far between. Most kernels are bug fixes or added functionality.