How long will each sprint take? And will it be the same for each topic or will it differ according to the subject?

On average, each sprint will take 2 week regardless of the topic. However, since the course is self-paced, you can choose to progress  through the material slower (if the topic is harder for you or you want to go more in depth into it) or faster (if you're already good at it or if you're certain that you will not want to specialise in that topic).