The default for a telnet server timeout or the keep-alive setting is 600 seconds.
The exit point QIBM_QTG_DEVTERM occurs when a Telnet session is ended by either a client or by any other means.
Use the command WRKREGINF to assign the exit program that you want to assign to the exit point above.
Alternatively, use the CHGTELNA command to change the attributes for the timeout.
Hope this helps.