75% of re-tweets happen within 1 hour of the original tweet

"There is something special about the time of day that a tweet is published. It is not that tweets happen to spread more quickly at certain hours but rather that 75% of all re-tweets happen within in the hour of the tweet. This means that you need to be cognizant of when your followers are online and viewing their own twitter accounts the most. Do you know when your followers are online the most? You can tell by examining their post frequencies."
Source: Research from Korea Advanced Institute of Science and Technology, reported by Going Social New, 10th May 2010
See the full research, based on analysis of 41m twitter accounts, and with lots more findings here (10 page pdf)