The idea behind this question is usually to know :

•           How fast you think you are?

•           Are you a realistic person? Suggested answer:

Answer that, while you will start doing all you can do from the first day onwards using all your organisation skill, management techniques and finding out the areas of improvements etc. yet to say about the time you need approximately six months to relect your contribution.

If there is a specific project to work on straightaway, you should reply that after two or three meetings with the staff you would be in a position to answer this question well, but you are sure to make a useful contribution in a comparatively short time.