When Do Subscriptions Expire in the Protected Content plugin?

This question is hopefully very easy to answer, but we can't figure out how long memberships last within the Protected Content plugin? Sometimes we create users and get an expiration email immediately, and other times it seems to take a lot longer. In any case, we can't find any settings for the membership length in the Help page or on wpmudev.org. We went ahead and used define( 'MS_LOCK_SUBSCRIPTIONS', true );, but it would be great to have more info on this so we can use the plugin on future sites.