The frequency of glass thermometer calibration depends on various factors, including usage frequency, operating environment, accuracy requirements, and historical calibration records. Below are some common considerations:

1. Usage Frequency
   Thermometers used frequently are more prone to wear and aging, thus requiring more frequent calibration. For example, thermometers used daily in laboratories may need calibration every three months, while those used occasionally may only require annual calibration.

2. Operating Environment
   Harsh environmental conditions (such as high temperature, high humidity, or corrosive environments) may accelerate thermometer aging, increasing the need for calibration. For example, thermometers used in chemical plants may require calibration every two months.

3. Accuracy Requirements
   In situations requiring high measurement accuracy (such as scientific research or precision manufacturing), more frequent calibration is necessary to ensure data reliability.

4. Historical Calibration Records
   If a thermometer has shown significant errors or instability in past calibrations, the calibration frequency should be increased.