According to a new study by social advertising and analytics agency Optimal, if your awesome new online campaign hasn't gone viral within three hours, it probably won't. According to Optimal:
Naturally, timing isn’t everything, as posts with attractive and relevant content always prove to have the greatest endurance. That being said, after three hours, marketers will probably have a good idea of whether or not they’ve created a winner.
Check out AllFacebook.com for the rest of the study.

A Graph.