If you are referring to the little text messages you get, and the little icon on the NavMap, that is controlled by (as an example):
[RndGroup 40]
GroupName=42a_Jap_Convoys_140
Category=0
CommandEntry=0
Long=15574430.000000
Lat=3929400.000000
Height=0.000000
DelayMin=8
DelayMinInterv=6060
SpawnProbability=34
RandStartRadius=0.000000
ReportPosMin=1980
ReportPosProbability=25
Heading=286.559998
Speed=9.000000
The ReportPosMin is the number of minutes between reports, while the ReportPosProbability is the percentage of chance that it does the reporting. So this is every 4th time through every 33 hours... any negative number for ReportPosMin would mean a "No report"...