Hi,

How can we make a konsole session freeze or timeout if it is kept idle for more than 15 minutes??

Not sure what you mean by freezing a session, you mean suspend the process running in the tab?

View → Monitor for Silence (Ctrl+Shift+I)
Toggles the monitoring of the current tab for lack of activity

By default, after 10 seconds of inactivity, an info icon will appear on the session's tab. The 10 seconds of activity can be changed in Settings -> Tabs -> Tab Monitoring

The type of alerts can be changed through Settings → Configure Notifications → Silence in monitored session.