i change the water once it turns back clear. using gh 3 part.
my thoughts are since the micro is heavily tinted, once the tint is gone from the res so are the micro nutes.
after a week my res is looking pretty clear, so i take that as a sign to change and re-up on nutes.
of course if oyu have 100 gallons and one plant...the water changes should last longer.
you should change the water once a week or every other, and top with durign the week with straight water. i dont like doing add-backs mid week.
you never know what the plants are using, so by doing add backs you are changing the ratio of whats available. and it seems availability in the correct ratio is key.
the ratios get all out of wack you start locking shit out. and even though there may be plenty of NUTE X in teh water, it may not be available.