mshEmailMarketer can send a maximum of 3600 emails per hour per client. (thats 1 email per second)

As an example, if you have 30000 emails to send at a rate of ~ 3600 per hour it will take approximatly 8.5 hours to complete the email campaign.

Note: email sending rates will vary depending on several factors such as the current server load, the size of the email your sending (including attachments and embedded images), and network load.