ASTM recommends a calibration of your gauge every 12 months. Calibrations are necessary due to the decay of the gauge Cs137 density source.
You should request a new calibration after a repair that involves electronics or GM tube replacement. It is the “drift” in electronics and the GM tubes that are also a top reason for needing a recalibration. Rough treatment of gauges can also impact calibration.
Any dispute between a gauge that has been properly calibrated vs. a gauge that is outside of calibration will generally be settled on the side of the properly calibrated gauge.
A properly calibrated gauge should work fine provided you perform timely standard counts. Take a standard count every day you use the gauge. Some manufacturers recommend several standard counts throughout the day, perhaps every few hours or as the materials being tested change.
If your gauge has not been used for over 2 months you will want to re-establish your standard count averages. To do this you need to take 4 new standard counts and then compare subsequent standard counts to the average of those 4 counts.