When to change out nutrient reservoir?

I currently am using a 50 gallon combined reservoir and Advanced Nutrients in my ebb and flow grow. Also using air stones and a recirculating pump 250GPH from one res to the other-they are connected by pass through fittings at the bottom. I have been dumping the res every 7 days, and mixing up a fresh batch. I never have any problem with algae. Tanks are kept covered. This is a 6" rockwool cubes layed on a coco mat grow. Is this too often or about right?

Thanks.

I use a 55 gal ebb and grow with canna nutes no air stone just a recirculating pump. I change the res as follows .. 3 weeks... then 2 weeks, 3 weeks... then flush in the 9th week. this method coincides with the additives i use with my nutes. I think 7 days on a 50 gal with Adv Nutes is retarded expensive. just make sure you top off every day and I think 14 days is sufficient in between changes.

I agree with the expense issue. As long as your water quality isn't going down due to exposure or temp changes, contamination etc. just make sure you're keeping the nutrient level where you want it. You may not be worried about the money now, but it sure adds up. The water and the nutrients. 14 days easily, especially with that large of a tank.

I am using 100 liters (26 gallon tanks) I change the water every 7 days. I am doing Aeroponics. No timers just spray 24hours with no signs of trouble so far.. I noticed I get a funky smell close to the end of the week too, I was thinking of changing water 2 times a week now that they are in flower.

how do you go 14 days with out the water getting rank? Do you add fresh water to the tanks as the plants drink up?? Do you add more nutrients and just watch you ppm?