This depends on the specification given by the service you are interacting with. I have worked with similar services and they have explicitly stated in their documentation that, In order to maintain the session valid I must poll the service at every few seconds by sending int "1".
However, If possible could please post the service name or any reference which we can read. If it is any company's private API then they must have described the use of the session id which they are returning.
Underlying technologies will take care of it, However if you want to persist those cookies then this answer for other question.
Persisting Cookies In An iOS Application?