Hi Nishant,
Backup job runs depend up on the data you are backing up. Normally for 100 GB data, a standalone SharePoint farm takes 5-8 Hours. This also depends on the configuration you have made in the server.
You have issue with backup using CA. its not a good sign that even after restarting, job is in process. You can stop this timer job by navigating to monitoring-->running jobs.
For backup, either use stsadm tool or SharePoint Management shell. This would be fast compared CA.
rijuv technical specialist,sharepoint