Mr. Chairman, you may not have to intervene at all. Of course, the witness has all the time he or she feels is needed to reply to the question.
Under the current system, when a witness is encroaching on a member's speaking time, he can only take it so far, but he prevents the other members of the committee from asking questions. If a witness is obviously talking about something else besides the topic raised by the question, I am sure that you will be able to intervene in an impartial and fair manner in order to direct the witness to address the questions that are being put to him.
I do not think that this poses a problem for the witness, since there is no time limit set on answering questions.