“How long would it take you to make a contribution to our company?” 

Again, be sure to qualify the question: In what area does the interviewer need rapid contributions? You are best advised to answer this question with a question: “That is an excellent question. To help me answer, what do you anticipate my responsibilities will be during the first six or seven months?” you give yourself time to think while the interviewer concentrates on images of you working for the company.