Hi,
Please take into account that the runtime of the backup mainly depends on hardware (tape drives, disk, system and I/O buses), the database and the system load, the configuration and the size of DB.
Were there any changes on the system recently? Have you also checked the system log and backup logfiles? Is there any specific job running or hihger load on the system?
Please also check the SAP Notes:
842240 - "FAQ:Backup strategy of large and highly-available databases"
13994 - "Long runtimes when saving with BRBACKUP"
23234 - "Backup of large databases with BRBACKUP"
Regards,
Bíborka