Public Portal in the Cloud does expire sessions similarly to how it works for self-hosted. I believe the default is 15 minutes and there isn't a way to adjust that. For the most part, session management should be invisible to the user and not something the administrator needs to worry about. Because Public Portal has a narrower use case, we can simplify some things. For instance, the main reason to want a specific expiration time is because it is specified in a security policy. But since there is no authentication for Public Portal, there is no real risk in having the session last too long. And since the licensing model is not based on simultaneous users, you don't have to worry about a lingering session keeping another user from the system.