F5 APM Inactive Timeout behaviour

You'll find the Session Update Threshold and the Session Update Window (assuming you're running v12.1.3 - you know how they like to rearrange items in the menu from one major release to the next...) here:

Access Policy => Network Access => Network Access List You should now see one or more network access lists. Select the one you need, then click on the "Network Settings" tab at the top and then look in the "General Setting" (Advanced) section.

Good luck!