From your dashboard, open the task by either clicking the task if it is shown, or clicking View All Tasks and then selecting the task you want to view.
On the task, you can see the time taken for task underneath the task score for each student.
To see the time taken for each question individually, you can click the tick or cross for that question.
This will bring up a detailed view for that question, where you will be able to see the time taken.
Alternatively, you can click View attempts under a student's name to go onto their task attempt and view the time taken for each question this way.