From the Glock Maintenance Manual
Your Glock pistol should be field stripped, cleaned, and lubricated as follows:
1. When brand new, before the first time it is fired, plus
2. at least once a month, plus
3. as required. This will be determined by the pistol's exposure to adverse conditions such as rain, snow, perspiration, salt water, dust, lint, etc. Additionally, the pistol should be cleaned and inspected if it has been dropped or if any defects are suspected or malfunctions experienced.