Thank you guys for your answers and explainations.

@Phil Probably 30-40% I guess.

@harpin Mainlanders and Taiwaneses have no trouble of understanding each others but there are some slight differences. Just like American English and British English.

„Well China is even worse (as an example of language variations) because what we call „Chinese“ is a set of languages which are as different from one another as the different Romance languages, they just happen to be all yellow on the map or something and they happen to be a unified political area, but there is no sense in which they are the same language. We call them Chinese dialects but that would be like calling French and Romanian two different dialects of the same language, Latin or something. I mean, these terms have no linguistic meaning, they have only socio-political meaning of a very complex sort and the way in which they interact with authority structures is crucial....“

Noam Chomsky, former MIT linguists Professor and world-renowned writer and intellectual. 1989 Television Interview. 

In my understanding. different people have different view. To Mainland Chinese. Cantonese is a dialect of Chinese official language(Mandarin). Because the characters are same as Mandarin and speak of the south part of China. To foreign Cantonese learners. they would think Cantonese is a fantastic language which they are interested. otherwise, any dialect is one language of others. Spanish and English are originated Greek and Latin. Anytime anywhere language is developing and changing during evolution. hope this makes sense. 

I want to learn more English, I speak Cantonese if you are interested in an English group, you can scan my wechat  https://photos.app.goo.gl/kcXXpyKv5byzKzT12