Companies want staff that can ramp quickly, but also want people who are realistic. So take into consideration how intense the job is and then give a good answer. For example, if you have simple responsibilities that don't require a huge development curve, then your ramp time will probably be shorter. If it's a complex set of skills that you need to develop, then your ramp time could be longer - the key is you have to explain why you believe that ramp time should be.