The ideal time duration required for comprehensive history optional preparation can vary from person to person depending on their prior knowledge, study habits, and dedication. However, on average, it is recommended to allocate at least 8-10 months for thorough preparation.

1. Understanding the Syllabus:
The first step is to carefully analyze the history optional syllabus provided by UPSC. It consists of various periods, themes, and topics that need to be covered. This will help in creating a structured study plan.

2. Collecting Study Material:
Gather relevant study material such as textbooks, reference books, previous years' question papers, and study guides. These resources will provide a comprehensive understanding of the subject.

3. Creating a Study Plan:
Devise a study plan that covers all the topics mentioned in the syllabus. Divide the topics into smaller sections and allocate specific time frames for each. This will ensure systematic coverage of the entire syllabus.

4. Reading and Understanding:
Start reading and understanding the topics in a chronological manner. It is advisable to begin with ancient and medieval history, followed by modern history. Focus on understanding the key concepts, events, and their significance.

5. Note-making:
While studying, make concise and comprehensive notes. This will serve as a quick revision tool and help in retaining important information.

6. Revision and Practice:
Regularly revise the topics covered to reinforce the knowledge gained. Solve previous years' question papers and attempt mock tests to assess your understanding and improve answer-writing skills.

7. Seeking Guidance:
If required, seek guidance from experienced faculty or join a reputable coaching institute for optional subject preparation. They can provide valuable insights, study materials, and guidance on answer-writing techniques.

8. Current Affairs:
Stay updated with current affairs related to history, especially those relevant to the optional subject. This will enhance the quality of answers and provide a broader perspective.

9. Time Management:
Manage your time effectively by allocating specific hours each day for optional subject preparation. Avoid procrastination and maintain consistency in your study routine.

10. Mock Interviews:
Towards the end of the preparation, consider participating in mock interviews to gain confidence and improve communication skills.

In conclusion, the ideal time duration for comprehensive history optional preparation is around 8-10 months. However, it is essential to customize the study plan according to individual strengths and weaknesses. Consistent effort, dedication, and a structured approach are key to achieving success in the history optional paper of the UPSC exam.