The answer to this question depends heavily upon the type and class of weighing scale that you are using.
If you have a high precision scale of Class I or Class II, then you should do the calibration every morning after you initiate the scale for the first time in the day. This is because, during the night, there would be temperature fluctuations that would affect the sensors on a minute level. Because the scale is a high accuracy scale used for Diamonds, research, gold, or silver, you would not want to take the risk of improper measurements, even if it is extremely minor. Hence keep an E1 or F1 weight handy
If you have a Class III scale, once a month or three months it’s better to check the weight, and if it’s not showing the right weight get it calibrated.
Class IV scales – Once every six months is ok and if the weight is not right, then get it calibrated.
Government Calibration is mandatory every year for electronic Scales and once every two years for certain types of Mechanical scales.