There are two reasons to flush.
ONLY TWO REASONS
Reason #1
You have a salt build up from continued and regular use of salt based nutrients.
Reason #2
You are dealing with an excess amount of nutrients that are causing nutrient toxicity.
Aside from those two reason, you do not flush and keep feeding up until harvest.
Flushing is a myth, it's bro science and has been scientifically disproven.