It can definitely be done naturally, but there's no general answer with regards to time frame. As your bodyfat gets lower, it obviously gets tougher as your body will start fighting back, trying to hold on to that fat. We haven't evolved to be 200lbs with sub 10% bodyfat; our bodies are very happy to store fat for survival, times of famine, etc.

Some people will be able to do it quicker, some longer, and some may never get there...