Hi Julie
I cannot speak about cost and effort, however the time to upgrade a large database from 15.4 to 15.6 should be as per my observations about 8 hours. This is very approximate time of the install run only, does not include any backups, and depends on the resources the server has and scheduling. After installs you would usually take some time and few users to check your basic functionality and ensure everything is working as desired.
Now as SAAS customer, there is a specific list created that may be helpful for you, I will get this communicated to you.