So when there is no audience parameter specified, the Access Token returned from Auth0 is intended for use with the Auth0 ‘/userinfo’ endpoint only. 
As I recall, you can’t currently set the default expiration for this from the Auth0 Dashboard.
To get an Auth0 Access Token for a specific API you must also specify an audience; from the example screenshots shared previously, the audience would be the identifier associated with the relevant API. You can then set Token Expiration to the value desired, and this will be reflected in the Access Token returned. Today, any Access Token generated by Auth0 can be used to call both the (custom) API associated with the designated audience and also the Auth0 /userinfo endpoint.
Hope that helps 