candba, think of sceanario after 18 hours of loading process and before your backup can start, the database crashed and you need to do media recovery, which in your case would be only import from last backup, and you need to reprocess all 18 hours loading process.

import also should take considerable amount of time, and ofcourse 18 hour plus.

Try to configure RMAN for the database and you can take Weekly full backup and after that can go for incremental backup. Test all your scripts on some test environment and implement on the production.

Dilip,