I know the present perfect continuous is used for activity which has stopped recently or now. When it combines with for, since, or how long an activity is done, it means the activity is still happening. I did a question on a grammar book which asked me to fill the bracket with the correct tense:
I (1) (read) the book you lent me and I will return it when I (2) (read) it.
The answers for the blanks are:
(1) have been reading (2) have read
I understand the answer for (2), but I don't quite comprehend the answer for (1). It uses the perfect continuous tense even though the activity is not stopped and it is not followed by how long the activity is. Or the clause (you lent me) indicates that the activity begins from the time the book was lent until now?