When to take part in an intercomparison is normally up to each laboratory to plan. An accredited laboratory though has to fulfil demands according the ISO/IEC 17025 which stipulates participation in a number of quality assurance activities, including participation in intercomparisons.
If there has been a major change in the laboratory’s activities, introduction of a new procedure, reference equipment etc, then participation in an intercomparison is normally recommended.
Historicalyl it has been very hard for laboratories in some calibration and testing fields to make these planned comparisons since there have been too few available.
The CFI (Centre For Intercomparisons) now offers a five year plan where each laboratory can plan their activities for the years to come.