As per the following KB I made changes that should have seen timeout reduced to 2 minutes.
I can't force Confluence to have a session timeout.
This issue has been reproduced on first a customers, my and then another engineers instance.
Disable all the 'Notifications' plugins that are in the Browse > Confluence Admin > Manage addons or Plugins click on 'Show System Plugins', next to the 'System Plugins'.
- Notifications and Tasks - Common Plugin
- Notifications and Tasks - Confluence Provider Plugin
- Notifications and Tasks - Host Plugin
Additionally, you will also need to disable the Confluence Quick Reload Plugin as well, to prevent calls to /rest/quickreload/ which is triggered around every 20 seconds