Two weeks if your plant is still dark green at the begining of the flush (or in organic soils), but one week is good if your plants leaves have begun to turn lighter green prior to flushing.
Adding carbohydrates (the sugars plants need for energy) such as Sweet, Sweet Leaf, Carboload, at LOW dose, can give your plant a slight energy boost to finish its biological process, without reversing the process of flushing your attempting. Or if you opt for a quicker method, you can purchase Sucanat for around $3/lb at your local natural food store. It is a form of dark-sugar with a molasses like consistancy, 1-2 tsp per gallon of water, and don't use regular sugars.