I've tried a lot of different flush methods. It depends on what you are giving the plant for nutrients (organic vs synthetic). With organic, you don't need to start the flush until the final week. With synthetics, I usually do 2 week flush. BUT, I also give it a light watering of molasses or some other carb for the first week of flushing, then pure water the final week.
If possible, your first flush watering should be about 2-3x the amount of water as a normal watering. This gets all the salts and built up nutrients out of the soil. This usually cuts flush time by a few days. Best of luck!