We analyzed open rates for just under 2,000 HTML email campaigns recently sent from MailChimp, and found that of the total number of people who would eventually open those emails:

• Nearly 1/3 will open within the first hour of sending (we’re all addicted to email, aren’t we?)
• The majority (53%) will open within 6 hours
• 78% will have opened within 1 day
• Within 1 week, 95% of openers will have opened
• And people will still open your email a full 30 days after sending (so don’t archive your hosted images too soon!)

Pretty interesting stuff, if you:

• Send breaking email news alerts
• Send email invitations to weekly events (give it a day or two for maximum effect, but it’s nice to know that lots of people will open within hours)
• Send followup campaigns to "those who did not open." Give people at least 1 week before you assume they didn’t open.

Finally, you should note that the Y-axis is not a "percent of list" but a "percent of those on your list who will eventually open." For example, if 30% of your list opens your email (that’s about average), this would show that 53% of that 30% will open within 6 hours.