To top up or not to top up, that is the question...

So my girls drink more then a litre a day, but they also feed heavily. My EC goes form 1,1mS on the day of res change to perhaps 0,8mS a day later, but then stays about the same as drinking rate and rate of nute intake reach a balance. This week I let it drink like that untill about half of the bucket was empty and then did a res change. That way I can get away with changing out the res every 6 days while they would be starving by day 3 or 4 if I kept diluting the nutes by toping up with fresh water (and I don't want to top up with nute solution as that may lead to toxicity problems). My question now is can lowering the water level (while keeping most of the root mass in water ofcourse) be damaging to the plant, or are roots OK kept out of the water for a day or two? Is the high humidity and splashing of the bubbles enough to keep those upper roots from drying out and geting damaged?

The larger they are, the less they are effected by the water level (and even nutrient strength/pH swings)... as long as the root don't get bone dry. I've even had plants that would empty buckets as far down as an inch of water or so between changes without issues. I doubt half-full is going to cause you any problems.

Thank you!