To create an unlimited membership or package, set the 'Number of Classes' to 999 in the setup field. This will effectively allow for unlimited access without specifying an exact limit.
Following that if you want that membership to have no expiry then create a REGULAR type membership and leave the expiry days blank.
If you want it to expire on a specific date, then create TERM type membership and specify a date on which the membership will no longer be recurring.