At interrogation failure it is written "

Does pc/npc know that interrogation failed or just thinks that he succeeded?

I have always taken "...you get nothing of worth" as the opponent has refused to answer, asked for his lawyer, said things the interrogator already knew and so on. So yes, I would say the interrogator knows that he failed the interrogation as the opponent refused to give up anything.

~ alemander

But that would mean that he can eventually interrogate the **** out of him sooner or later.

Interrogation, as described in the rulebook, definitely can and does include psychological torture and can include physical torture. Functionally everyone will eventually break under torture, if the torture doesn't accidentally kill them first.

Anyone will say anything under sufficient torture. Anything includes, along with a great many other things, the truth.

It may take a very long time to get to the truth, and separating out the lies from the truth can be difficult, if not impossible.

If you're worried about just repeating Interrogation tests until they get all the answers they want/need, remember, it is only 1 answer per DoS. An Interrogation test imposes a level of fatigue on the subject (which means they'll loose consciousness eventually, and usually after 3-4 Interrogation tests), and each Interrogation Test takes 1d5 Hours. And, if the Interrogator fails the Interrogation test (not just looses the Opposed test), the subject takes 1d10+Interrogator Willpower Bonus damage, presumably reduced by Toughness Bonus, if they fail by five or more degrees of failure, they do the damage and the subject gains a +30 bonus to the opposed test.

In a well equipped facility, and with good Interrogators, the risk of doing damage (and thus accidentally killing the subject) can be negated. However, each Interrogation attempt still takes 1d5 hours.

So ... yeah, eventually you can wring somebody dry. Key word being "eventually".