Let me start by saying this; '' everybody do think!''. As a student you think on how to be successful academically, you think during tests or exams about you have read and had to memorize, you think when you are trying to solve a complex mathematical Question and so on. That thinking that you as a student you are engaging your brain is known as cognitive task. That is to say the act of processing your thought.
When you multitask, it means you are doing more than one work at a time and this is due to humans' ability to perform Subconscious cognitive tasks.
It should be thirty minutes three times a week for it to be beneficial. But if you want to go with a stronger goal, it should be four to five times a week.