The only way would be to test it out, if you know the time for the backup, then you can time the restore on the new server.

There is more to migration than restoring the database, the indexes and statistics need to be updated, all jobs / logins / linked servers / database mail / Server wide DDL triggers etc etc all have to be recreated.

Which is better again test it out. My preferred method is backup and restore, its tried and tested.