When it comes to a job interview, it is vital that you put yourself in the best light in order to stand out to your potential employee.
Therefore it would be advisable to say that it will not take you that long before you make an impact to the company, although you will need to make sure that what you say is realistic. I don't think your interviewer will believe you if you say you can make a difference in a matter of hours!
Once you give a realistic time frame, you will have to explain yourself. Put forward some of your best qualities, such as that you are motivated to reach targets or devoted to meet deadlines.
Depending on the role you are going for, you could always offer some potential ideas on how you think you could make a contribution to the company. This will show initiative and make you stand out from other people trying to get the job. Even if the interviewer doesn't like your ideas, they still will be impressed that you are willing to make suggestions already.
The aim of this question is to put you on the spot, but as long as you stay calm and think through your answer, it is actually a good way to promote yourself to your potential employer.
I wish you the best of luck!