Well as a general rule you should clean your guns once a year even if you aren't using them. Otherwise you should clean them every time you have them out. One thing that people don't remember is that once your bore is clean you need to run a patch with oil down it to keep your bore lubed and rust potential low.

For specifics follow the maintenance guide in your owners manual.

One easy way is to take your Glock to a certified service center and they will take care of it.