Calibration Intervals – it is generally accepted that the interval of Calibration of measuring equipment be based upon stability, purpose and degree of use.
Stability means that your measuring equipment has the inherent ability to maintain its metrological characteristics overtime and use.
The purpose of your equipment is of prime importance, in general the more critical the application or measurement being made, the more frequent your calibration cycle needs to be.
Usage is pretty self explanatory, but it simply means how often you use the device to take measurements coupled with the environment it is taken in.
You shorten the interval/frequency if your previous records show that the usage indicated the need to tighten up on the interval. By this I mean, suppose your item, item A has a 1 year cycle it is calibrated and found 'out of tolerance' (OOT). You calibrate again at the 1 year interval and it again is OOT. So you shorten you calibration cycle to say six months and on return from calibration your 'as found' and 'as left' reports show in-tolerance. Your next calibration cycle reports the same thing; you know you have probably found a happy place for your equipment.
At my work place the calibration cycle can be extended or shortened based on well reasoned and documented evidence, but only after we have five (5) consecutive, 'as found/as left' in-tolerance reports from the Calibration facility on record. Then we only ever bump the cycle by three months, taking into consideration the three criteria stated at the beginning of my reply.
Cost is also a reason for looking at these cycles but it can not be the sole determining factor, after all this is the cost of doing and staying in the business we are all in.
In short the choice of calibration interval is basically a well reasoned and documented trade off between risk and cost. Too great of an interval increases the risk of mistakes and too short of a period increases the risk of needless spending. The greatest/best advice you can obtain is from the OEM manual supplied with your test equipment in the first place, and then from this you apply the other determining factors.