Where is the time limit set that causes an IMS session to be cancelled if the IMS user of that session has been idle for some 'set' time ?
The IMS OPTS record specifies IDLE(IDLE) - this means that the logonid field IDLE will contain a value (in minutes) that will be used for that user only.
If a user's logonid has IDLE(120) that would equate to an idle time of two hours.
Review the logonids and see what values are in use.
for example you could run an ACFRPTSL report for all logonids with an idle value greater than 60 minutes...
//STEP1 EXEC PGM=ACFRPTSL,REGION=4096K
//SYSPRINT DD SYSOUT=*
//SYSIN DD *
IF(IDLE > 60)