There's no set answer to that. It depends on what clouds you're using and moving to, as well as how much data you need to move. I've done a few email migrations for my small company and one big help desk migration. It all took about a week. Most of the time was spent communicating back and forth with the migration vendor and getting the transfer just right. The data movement itself was relatively quick. But moving that 27TB I currently have in cloud file storage would definitely take quite a long while, no matter what service is used.