I flush for 2 weeks using only purified water. I can't really compare my results to any others because I have only done it this one way. Perhaps next season a flushing experiment is in order. *Shrugs.*

Oh and be cautious when keeping them in the dark for the last week as it increases chances of them catching a mold or mildew. This late in your grow mold would be devistating. Make sure your grow room is nice and dry before putting them in the dark.