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.