The Old Reader determines how often to fetch new posts from a feed by looking at how frequently new posts are published to the feed. This may be as frequently as every 10 minutes and as infrequently as once a day.
Premium subscribers get a boost to this time of around 70%. So if the feed is normally fetched for free members at once every 100 minutes, a premium subscriber's feed would be updated every 30 minutes.
If a Premium subscriber and a free member are subscribed to the same feed, then the free member will benefit from the faster refresh times of the Premium subscriber.
Feeds that support Pubsubhubbub get updates almost instantly. For more information on Pubsubhubbub, visit this page: https://code.google.com/p/pubsubhubbub/