If this is the frist time backups were done on an SSB it is expected behavior that the backup could take more than 24 hours.
Continue to monitor. If an issue is suspected open a service request with support and include a debug bundle:
From the SSB Web GUI -> Basic Settings -> Troubleshooting: Click "Collect and save current system state info".
This will automatically enable debug mode, create a debug bundle and display a window in which a location can be designated for the debug bundle to be saved.