Edit Workday Account - Task allows you to set the Session Timeout Minutes Enforced for an individual user.
How ever, some times even though you enforced it takes a default of 15 min or asks you to key in between 5 min to 20 min. The one possible reason for this is that you being a Security Administrator, you might have added all the user based security groups to your id [Assign User-Based Security Groups for Person]. There is a security group which restricts this mostly - Credit card Administrator.
Which ever security groups have access to the Domain - Manage: Credit Card Data will not have more than 15 min of time out minutes, you can not override beyond the limit.
Remove this Security Groups if you are not using really then you should be comfortably override this.