(like) water off duck's back

The above idiom means one is not insulted or upset from criticism from someone else. If I say the bad opinion about me is like water off duck's back, I mean I am not insulted or upset by the someone's rude remarks.

If one wants to write something in English, they need vocabulary to use in their writing. Therefore, it is essential to have enough amount of vocabulary in one's head. 

But it also depends on what they want to write about. If one needs to write an academic paper they need to know a range of sophisticated and highly advanced English vocabulary, let alone using them appropriately. But if one wants to just write about their daily life they need not have as complicated and advanced as the ones mentioned above.

How much vocabulary would I then need to know in order to complete my course in the University of Adelaide? 

I just finished vocabulary test suggested by someone on EnglishClub.com and got 7,590 words as a result. The numbers of vocabulary given was not quite satisfactory to me, but I accept it with all my heart. I want to expand my vocabulary range just not because I need to graduate the university but I want to be very fluent and proficient at the English language. I would like to speak English as good as someone raised and educated in an English speaking country. 

I believe if I do not give up on expanding my vocabulary by reading, searching their definition, memorising, and using, one day I can call myself speaking English as good as those ones who have been raised and educated in an English speaking country. 

It is my desire and my goal in the course of my life.