Both "extreme" and "extremely" can be used in an absolute or relative sense. In the absolute sense we are talking about something that possesses a particular property to the greatest degree possible.\, i.e. there is nothing else, that we know of, that possesses that particular quality to a greater extent. eg:
The Burj Khalifa is extremely tall.\, meaning that it is the tallest building in the world.
Challenger Deep, in the Mariana Trench is extremely deep, meaning that of all the oceans on Earth, Challenger Deep is the deepest point.
When used in a relative sense the words "extreme" or "extremely" really mean "to a great extent" but not "to the greatest extent possible". It many cases they can be replaced with "very". e.g.:
People stayed home because of the extreme cold.
This just means that people stayed home because it was very cold. It does not mean that people stayed home because the temperature was zero degrees Kelvin, which scientists regard as being the lowest temperature possible.
The Empire State Building is extremely high.
As mentioned above, the Burj Khalifa is the world's tallest building. The ESB is currently rated as the 40th tallest, but it is still very high.
Using "extreme" and "extremely" in their relative sense is probably the most common way in which those words are used. As a consequence, when someone uses "extremely" (or even "extreme") in a sentence, it is not illogical for someone to wonder about the degree of extremity. e.g. if you say:
I am extremely afraid of snakes.
I might be curious regarding how extreme your reaction might be if you saw a snake. Would you:
- Drop dead from heart failure. (yes, I am aware that if this was true, and you had previously seen a snake, you would not be able to talk about it.)
- Become catatonic
- Run away screaming
- Experience an increased heart rate and heavier breathing
- Back away quietly to avoid notice
I could just throw a snake down in front of you, and watch what happens, but it would be more polite to simply ask you.
I could ask:
How extreme is your fear of snakes.
or, especially in conversation, I may just repeat the words that you had just used, e.g.:
How extremely are you afraid of snakes.
This sounds odd to a native speaker, so I would probably ask either:
How extremely afraid of snakes are you.
How afraid of snakes are you.
The words in italics are a direct quote from you and I would probably place an emphasis on these words to indicate this. Of these last two questions, the first is probably not grammatically correct, but I could easily imagine it being said by a native English speaker, especially in the middle of a conversation. The second is grammatically correct, but it may just illicit the answer, "Extremely', which would not progress the discussion in any way.