1% or 10 per 1000 is considered average.
Generally it doesn't matter how fabby dosie your leaflet is, it's all about it landing on the doorstep on the very day that someone is thinking about having something done. Although that is not a strict rule and I could give 2 examples of why it isn't, generally it's all about timing.
Mike Halliday is the King of leaflets speak to him, although be prepared to put out about 10,000 a month, hitting the same streets month after month.