How quickly are Broadcast messages sent?
Broadcast newsletter messages are generally sent within 30 minutes of the time they are scheduled to send on, for all but the largest of subscriber lists.
Historical statistics show that over 96% of all broadcasts are completely sent within 10 minutes. Before asking about the status of any broadcast please allow at least 2 hours to account for extremely high volume, which occurs occasionally.
Pro tip: The peak sending time is at the top of every hour (9:00am, 10:00am, etc). The lowest sending volume time is the 10 minutes immediately before the top of the hour. If it's critical for your message to arrive at the top of the hour, schedule it in the period right before the hour. When looking at aggregate send times, there is zero correlation between what part of the hour you send and overall open and click rates.