The idle time for a role is not calculated. Roles time out when their user's session times out.
As secadmin at label admin_low
in User Manager, select a user, not a role.
Click the Idle... button.
Press the Idle button labeled 5 mins.
Choose a convenient setting in keeping with your site security policy.
The options are to lock the screen or to log the user out; different time lengths are possible.
Click OK, then Done.