[genericuser]

It really depends on what type of relationship you have to the person asking it in the US in my experience. A stranger or a person you know in a professional manner and yes social doctrine seems to dictate you should reply with a positive remark. But while it is still intended as a greeting in more friendly situations, it is quite common for me to give and for people to give me vague but non-positive answers. For instance 'shitty', 'uhg don't even want to talk about it', or just staring with dead eyes and shaking your head no (while I don't actually know that this last one is acceptable, I use it a lot and love it, and no one seems to give me shit about it).

What seems to be unacceptable and would probably be in German also is just completely unloading with all the problems of your day in detail. Basically keep it to under like 10 words and it is an acceptable answer among friends, at least in my experience.