You get an update

How often should I update my Linux server.

I run apt-get update -qq; apt-get upgrade -duyq daily. This will check for updates, but not do them automatically.

Then I can run the upgrades manually while I am watching, and can correct anything that might go wrong.

Besides the security concerns of maintaining a patched system, I find that if I leave it too long between patches, I end up with a whole bunch of packages that want to be upgraded, and that scares me a-lot more than just upgrading one or two every week or so. Therefore I tend to run my upgrades weekly, or if they are high priority, daily. This has the added advantage of knowing which package broke your system (ie. if you're only upgrading a couple at a time)

I always upgrade less critical systems first. I also have a "rollback plan" in place in case I can't fix the system. (since most of our servers are virtual, this rollback plan usually consists of taking a snapshot before the upgrade that I can revert to if necessary)

That being said, I think an upgrade has broken something only once or twice in the past 4 years, and that was on a highly customized system - so you don't have to be TOO paranoid :)