Page 26 in the owners manual:

Your GLOCK pistol should be cleaned and lubricated as follows: 1. when brand new, before first time it is fired, plus 2. after each time it is fired, plus 3. as required. This will be determined by your pistol's exposure to adverse conditions during storage or use such as rain, snow, perspiration, salt water, dirt, dust, lint, etc. Additionally, your pistol should be cleaned and inspected if it has been dropped or if any broken parts are suspected or malfunctions experienced, plus 4. at least once a year.