That is not a highly difficult answer. But the answer is not completely straightforward, either.
You need to pick a reasonable initial interval. If there is an interval recommended by the manufacturer, that should be the first interval you use. Then monitor their historical ability to stay within your maximum calibration tolerances. Using that historical information, lengthen the interval until they have the percentage probability of remaining in tolerance that you feel comfortable with. This interval truly could be anything from as short as three months to perhaps as long as two years. But you must use either historical data (I described above) or manufacturer's recommendations.
It would not be prudent of me to state a particular generic interval, as it may not be adequate for your use, or for the particular model transmitters you are using. The object is not to find only the most cost-effective, but the most cost-effective that still maintains your desired probability that the units will remain in tolerance for that interval.
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