@srinivas_chann1 There is no OOTB way to get the list of the workflows which are not used in the system for a duration of time.
Reason - AEM have the "Purging of the Workflow Instances" which is done via the configuration i.e. "Adobe Granite Workflow Purge Configuration" - to purge workflow instances according their age and status. You can also purge workflow instances of all models or of a specific model.
So with this, you will not be able to get the list of the completed/suspended/terminate workflows if they have been purged by the above mentioned config based on how the configuration is configured in the system
And it is even not recommended to disable the purge as that you increase the number of the nodes stored in the location i.e. "/var/workflow/instances".
The best way is to analyse the business requirement and remove the custom workflow which are not required now.
Hope this helps!
Thanks