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

It is not obligatory to fill in the questionnaire in one go.

While answering your questionnaire you can always go back to the Tasks page or to any other page.

Note: if you close the page or go back to Tasks, the answer to your current question will not be saved. Answers to previous questions will be saved automatically. 

In the Tasks tab, you can find your progress. The progress bar will be displayed on the Questionnaire icon.

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