When you are inactive in the system for 20 minutes you will receive a pop-up notification that you have been timed out.
If you press "I'm still here" the counter will reset.
Please note: This applies to all tabs/browser windows you have open, not just the one you are actively working in. If you inactive in any one tab for 20 minutes, the notification will appear in THAT tab. If it times out, you will be logged out of all tabs.