Do I have to fill in my questionnaire in one go?

In the app

No, while answering you're questionnaire you can always go back to the TO-DO's page or to another page.
Note: if you close the page or go back to TO-DO's, the answer to your current question will not be saved. Answers to previous questions will be saved automatically. 

In the TO-DO's tab you can find your progress.

On web

No, while answering you're questionnaire you can always go back to the overview or to another page.
Note: if you close the page or go back to overview, the answer to your current question will not be saved. Answers to previous questions will be saved automatically. 

In the overview you can find your progress.

Note: if you close your questionnaire before completion be sure to continue completing it before the due date.