I want to change the default SSO session length from 8 hours.
The default session length is documented here https://devcenter.heroku.com/articles/using-single-sign-on-sso-services-with-heroku-for-end-users#q-how-long-does-my-sso-session-last-on-heroku. After initial sign in, we have an 8 hour session. This is a security feature that prevents long-running actions for a user that could be removed from the IDP without our knowledge. 8 hours is not configurable at this time.
Changing this is not possible at this time but we recommend subscribing to the Heroku Changelog. This will let you know about updates, improvements and deprecations ahead of time so that you can make any necessary adjustments.