Sure! Here are the sentences transformed into the required structure:

1. They used to go to school in half an hour.
- It **took them** half an hour to go to school.
- They **went** to school in half an hour.

2. We often spend 30 minutes reviewing our lesson before class.
- It **takes us** 30 minutes to review our lesson before class.

3. It took him twelve hours to fly from Hanoi to London.
- He **took** twelve hours to fly from Hanoi to London.

Let me know if you need any more help!