When planning your cloud migration, don't forget to allow time to move the data from your current provider to the cloud provider. Email and document management systems grow significantly over the years, and your data may take longer to move than you think, even if everything goes smoothly.
We clocked these benchmarks on a recent migration:
Duration and Speed - Exchange on-premise to Office 365
Duration and Speed - IMAP to Office 365
Many factors go into the speed of the migration, such as the speed of all network connections you are using and the underlying hardware and network performance for on premises implementations.
You also should consider the time devoted to monitoring the migration and restarting failed processes.
Some cloud providers offer shortcuts to migration which allow you to send hard drives to their facilities to eliminate the Internet performance bottleneck, but this requires advance planning and coordination, especially if you do not want to have the systems run in parallel.
Migration tools provided by cloud vendors and third parties can simplify your work, but at the end of the day the data needs to move from one place to another, and this inevitably takes time.