When planning to migrate on premises databases to V12, the size of the database is a huge factor in how long the migration will take. The export of the database, the transfer of the data, and the import will all increase in proportion to the size of the database.
Another big factor in the restore/import time when moving your databases to V12 is the performance tier you are restoring too. The restore/import process requires a lot of horsepower, so to help expedite your migration, you should consider restoring to a higher performance tier. When the database is online, you can easily and quickly drop down to a lesser tier that meets your daily performance needs. Being able to change performance tiers with a few mouse clicks is one of the big benefits of Azure SQL Database.
There are some design considerations for moving to Azure SQL Database, and once those are covered, Tim’s article helps with the actual migration process.