Can I edit the time for all the questions in an Assessment/Quiz at once?

Yes, you can! When you edit an Assessment/Quiz, 

1. Find the time setting and click on it. It will be located at the top of the questions
2. Use the drop-down to schoose the desired time duration for all the questions
3. Click on 'Update'
4. For the changes to reflect, click on 'Publish'