Can I use present perfect in questions when the person is still waiting or has stopped waiting? I mean in both cases.
A: "You are still there. It's unbelievable! How long have you waited for them?" The person is still waiting.
B:"So you are on your way home. "How long have you waited for them?" The person is no longer waiting.
Present perfect relates to things that started in the past and have some effect that lasts until the present. Sentence A is correct because the person is still waiting. Sentence B is incorrect because the person is no longer waiting - no lasting effect.
A) "How long have you waited for them?" is OK, although I feel that we might be more likely to say "How long have you been waiting for them?" if we knew that they were still waiting and that it had been some time.
B) "How long have you waited for them?" doesn't work once the person is on their way home. It would work with "So, you are about to leave" or "So, you are ready to give up". It might just about work after "So, you're leaving now". But in your sentence it should be either "How long did you wait for them?" or "Have long were you waiting for them?". Both are fine.
