Opt-in emails e.g., newsletters, emails from retailers are far more likely to be opened within the first hour after delivery than at any hour after that, according to GetResponse findings released in October 2012. Some 23.63% of all email opens occur within the first hour after delivery, and 9.52% in the second hour. Another 6.33% occur in the third hour, and 4.8% in the fourth. Five hours after delivery the results drop more than 90%, and less than 1% of opens occur 24 hours later.

via Opt-In Emails Get Most Opens Within First Hour Of Delivery, But Timing Counts.