The half-life of a Facebook post is 30 minutes

"A Facebook brand post will get half of its reach in the 30 minutes after it is published, according to updated data provided to MarketingCharts by Socialbakers. Socialbakers analyzed a select group of brand posts published on October 31st, finding that an average of one-third of post reach was obtained in just the first 10 minutes after publication. Previous data from Socialbakers has indicated that after the first half of reach is obtained in the 30 minutes post-publication, the rest is attained in the following 7 hours.
Socialbakers’ data shows a dramatic drop-off in share of reach after the first 10 minutes, when posts got one-third of their total reach. In the 11-20 minute period after publication, posts reached an estimated additional 13.2% of their total audience, while in the following 10-minute period, they reached just another 4.7% of their total audience. By the time 90 minutes had passed, the average post was reaching less than 2% of its total audience.
This drop-off is even more pronounced than email opens. According to a GetResponse study released in October, some 23.6% of email opens occur within the first hour after delivery. By that time, a Facebook post would have already reached 60% of its total audience."

Source:  Marketing Charts, 2nd November 2012