Your fastest approach is to just query for it. There isn't any additional processing on expiration date (beyond formatting) to necessitate going through APIs for it. If you still want to do this through API you can get a list of a users active subscriptions using the below.
Active Subscriptions:
Code:
$subscriptions = cbpaidUserExtension::getInstance( USER_ID_HERE )->getActiveSubscriptions();
If you also want to get all subscriptions, even those expired, then you can use the below.
All Subscriptions:
Code:
$subscriptions = cbpaidUserExtension::getInstance( USER_ID_HERE )->getUserSubscriptions();