It is used to express an unfinished action that started in the past and continues until now. For and since are frequently used with present perfect continuous:
How long have you been reading this book?
I have been cooking for 2 hours now.
They have been fighting for years.
It is used to express an action in progress that started in the past and has continued until recently or until now. The action may have just finished and we can feel, hear, or see its result in the present:
Have you been smoking? i can smell a cigarette
The ground is so dry. i has not been raining for months.
It is used to express an action repeated over a period of time. Sometimes it is called a temporary habit which started in the recent past and continues until now:
I have recently been drinking 2 liters of water every day.
They have been spending a lot of money lately.
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ATTENTION!
WE DON'T USE STATIVE VERBS IN PRESENT PERFECT CONTINUOUS