Per the owners manual, Glock recommends cleaning (and relubricating) a gun:
- When New
- After Shooting
- At Least Monthly
- As Necessary based on exposure to adverse conditions such as rain, snow, perspiration, salt water, dirt, dust, lint. etc. Additionally, the pistol should be cleaned and inspected if it has been dropped or if any defects are suspected or malfunctions are experienced.
For what it's worth, if a gun is being stored in a climate controlled area with relatively low humidity and no other adverse conditions and is not being handled, monthly cleaning is probably overkill. For the conditions you describe, monthly cleaning makes good sense.