I know this is a real newbe question but. I have upgraded three versions in 90 day's. Each time it is for some sort of security upgrade. Its a simple deal download the rpm's and within minutes it's done. So now when I reboot my machine I get the option to boot into several different kernels.
Are you suppose to uninstall the old version's or do you just let them stack up ?
I've not upgraded my kernel for ages - still on 2.4.3 at the moment, since the later 2.4 kernels are such a pain. At work it crashed our production servers every few days and we always have to ensure they have the latest patches. Maybe I'll switch to xBSD sometime.
Normally you don't need the older kernel RPMs if the newer one is working. However, it is always nice to keep one older kernel just in case there's a glitch in the new one (which happens quite often in the later 2.4 kernels).
As of the frequency of upgrade, it all depends on individuals. Usually I won't do an upgrade if I know the security bug won't affect me, for example the bug only affect one of the kernel modules that I don't use, or it's only a local exploit. I rather stick to something that works than going for the latest and greatest (and buggiest) kernel release.