You have put the negative in twice. Other than that it is correct: "How long have you been waiting for a bus? I have already been standing here for fifteen minutes but no bus has come" or "How long have you been waiting for a bus? I have already been standing here for fifteen minutes but no buses have come" or "How long have you been waiting for a bus? I have already been standing here for fifteen minutes but a bus has not come yet"

How long have you been waiting for a bus? I have already been standing here for fifteen minutes but no bus has come.

How long have you been here for a bus? I have already been staying here for fifteen minutes but no one has come.