I have been studying English...

[suprunp]

"I have not been studying English for a long time." - It literally means that I have not touched any of the English textbooks, I have not read a word in English, I have not spoken in English etc.

"I have not been studying English seriously for a long time." - I have been studying English for a long time, but I have not been taking it seriously. I has been more like a casual activity to me.

"I have been studying English seriously for not a very long time." - I have been studying English; I have been doing it for some time (apparently for a short period); I have been taking it seriously.

Q: Does each of these sentences sound correct and mean what I've assumed?

Thanks.

[5jj]

"I have not been studying English for a long time." - It literally means that I have not touched any of the English textbooks, I have not read a word in English, I have not spoken in English etc.
Not generally (though it's a possible meaning). It would be clearer to say: I have not studied English for a long time.

"I have not been studying English seriously for a long time." - I have been studying English for a long time, but I have not been taking it seriously. I has been more like a casual activity to me.
That's possible.

"I have been studying English seriously for not a very long time." - I have been studying English; I have been doing it for some time (apparently for a short period); I have been taking it seriously.
That's not natural. Better: I have been studying English seriously for only a short time.

Incidentally 'for some time' usually means 'for more than a short time'.