An exact estimate is difficult because of the many variables involved. ShuttleCloud uses the fastest servers available, however, and we are only limited by the bandwidth that your email server allows.
A migration of a few thousand emails is typically complete in a few hours or less. Under ideal conditions we transfer anywhere from 250-750 MB per hour. A migration of tens of thousands of messages my take less than a day and a migration of hundreds of thousands of messages usually requires a day or two to complete.