I agree with what Sataincpuppy said. To answer the how often:
- Set a regular schedule that works for your company.
- Consider watching a security page for any critical vulnerabilities that might require updates sooner than your normal schedule.
With updates you must always know how to roll back as well, so make sure you know how to select the previous kernel in grub as the default if for some reason the update gives you problems.
Also, don't just worry about the kernel, for instance, if this is a Apache web server you better keep an eye on those updates as well.
RE Repo For Security Updates:
The default repos shold be fine unless maybe you have special high security needs. If you only want security updates just only use the security plugin:
yum update --security