In my experience, export/import is efficient from MB to several GB (10GB) and not more. The export and import process in this case takes 1 to 3 hours, depending on the strength of the Oracle machine. Usually when you do export/import, you move your schema to production and this process takes time. In cases where the database is bigger than several GB (Terabytes), don't use export/import, use cold backups.
This was first published in July 2001