Moritz said that a month was ideal, in most cases. I generally wait three weeks, minimum, but am beginning to think a month would be better. If you follow his advice to have a colonic afterwards, it takes a whole week just for your bowels to normalize, let alone other things. I don't believe I could manage doing them every two weeks. Remember that he suggests almost a week of apple or sour cherry juice (or malic acid) beforehand. So if you do it every two weeks, you are basically always preparing for or recovering from a flush.
In my reading this forum, my impression is that those who recommend, or do, the flushes every two weeks (or oftener) do so because they are suffering pain specific to the liver or gall bladder and are trying to relieve it.