I would still give at least a full hour for the migration, then the amount of time you will need to test your sites before making DNS changes.
I think telling your members about 2 hours of seeing a maintenance page is likely and more of a safe estimate.
You won't need to put the site in maintenance mode until right before the migration starts. So at the time you give the go ahead to proceed that is when you should put the site in maintenance mode and then we will start transferring the accounts.
After the migration is finished we'll provide you with details on how to test your site as well as the DNS changes to be made.