Past Perfect tense without any temporal words?

Hello, teachers

I’m worried about a question from my textbook.

Question: Translate the following into English, beginning with “from what time”(I don’t copy the original Japanese sentence here.)

The answer is: From what time had you been studying?

There are no other sentences that tell the situation. The original Japanese sentence is a short one, and so is the answer.

Could it be possible to use past perfect tense without any words telling time?
If so, what does the sentence above indicate?

Thanks in advance!

The answer should have an indication of time.

I’m not sure why you have chosen to use ‘had’ rather than ‘have’.