Do I need to use Two-Factor Authentication every time I want to login?
Short Answer: Yes. Will you be prompted every time you log in? Depends on what device you are trying to log in on.
If you are on your own device you can save the device for 30 days by selecting remember this device for 30 days when you are at the second authentication step.
When you select remember this device for 30 days it saves a cookie in your browser. So if you delete your cookies, clear your browsing history or use another browser you will need to go through the Two-Step process again.
If you are on a lab computer, friend's computer, or any other device that is not owned by you then yes, you will need to login every time.
It is highly recommended that if you are NOT using your own personal device or work computer DO NOT check remember this device for 30 days.