
Now onto the question...
Answer: Post whenever the most recent status update for your page stops showing up in your fans’ News Feeds.
If you post often, you will see an immediate spike in News Feed impressions, but it’s generally not worth the cost in lost fans. When your fans see two status updates from you in their News Feeds, they’ll likely get annoyed, and will consequently unsubscribe or un-fan. There are few exceptions to this rule.