Wadde

[woven hawk]

Is it possible?
Yes
Do they have an expiration time?
Nope
Can we create multiple customer portals for reuse.
Yes
How to these API created customer portal configurations affect the default customer portal configuration which is configured in the stripe portal?
It has no impact, those configuration are taken into account only when you specify them when creating a Customer Portal:
https://stripe.com/docs/api/customer_portal/sessions/create#create_portal_session-configuration
Is it possible to delete a customer portal configuration? The API only expose an update endpoint.
Nope, you can just de-activate it:
https://stripe.com/docs/api/customer_portal/configurations/update#update_portal_configuration-active