The portal access URL that is generated using the portal session API is valid for 1 hour from the time of creation (in fact the API response JSON contains the expiry timestamp as well). 

• The access URL should be accessed by the customer within one hour from the time it was created.
• Once accessed, the session is valid until the user logs out from the portal UI or logout API is called from your application for this session.
• Once accessed, the access URL cannot be reused. Hence do not persist with this URL. Whenever you need to provide access to the portal, you need to create a new portal session.