The short answer is that it depends on how many other email campaigns are being sent at the time your campaign is launched. Here's how it works:
Scheduled for delivery
When you launch an email campaign it is added to a sending queue. The status of the campaign is showing as "Scheduled" on the messages list page.
Sending usually begins within a few minutes but during busy periods sending may be delayed for several hours. Large spikes in email activity are unpredictable, however, history tells us they only happen once or twice a month.
In progress
Emails are being sent when the status shows "In Progress".
Sending rate is approximately 50,000 emails per hour but this is also affected by the volume of emails being sent at the time.
AB test in progress
There is a two hour delay while results are collected from test groups when you're running an AB test.
Sent
Once all messages in your campaign are sent, the Status column on the messages page will display "Sent".
Related article: all message and template statuses.