Question

How to return a postponed task to the general queue if it hasn't been processed by a certain time?

Answer

In FlexiCapture 12 this happens automatically after the specified timeout for postponed tasks or tasks assigned to specific operators has passed: https://help.abbyy.com/en-us/flexicapture/12/admin_guide/timeouts_tasks 
The Administrator is able to specify a timeout period after which a task will be automatically returned to the general queue. Timeouts can be specified for the following types of tasks:

• Postponed tasks
  An operator can pick up a task from a queue and then decide to postpone it. Postponed tasks of one operator will remain inaccessible to other operators, which may sometimes delay the processing of the entire batch.
• Postponed rescanning tasks
• Tasks assigned to specific operators
  Tasks assigned to one operator will remain inaccessible to other operators.