It takes awhile because of the toxic salt that the plants give off. When I put in fresh rez it looks like purple, brown kind of. ( lucas formula ). And when I take it out in 10 to 14 days it looks just like piss yellow and almost has the same TDS. I only top up with RO water. I use more nitro to veg with too. I have small rez's but maybe a big one can go alot longer.