I'm totally new to hydroponics but have been researching heavily on the web so take this for what it's worth. Somewhere I read the salts and minerals that the plants don't use may build up over time to levels harmful to the plants. Flushing/changing the solution gets rid of these build ups.

Mike

There is no set answer because situations vary-
- liters reservoir per plant
- growth rate of plant
- vegetative vs. fruiting stage
- disease in reservoir
-other factors: plenty if we want to think more

If you are looking for other practical rules of thumb I would add, when you have added double the amount of full strength of nutrients. This is because fertilizer ppms for each individual nutrient are not reflected on a TDS estimates with the testers so TDS starts to be a less meaningful as far as the fertilizer formula actually in the res. Plus I change every 5-6 weeks to keep things disease free, living here in mildewy Florida. I usually make one small fertilizer addition at 4 weeks and then run it down till change (my way of economizing somewhat since I want my plants to clean their plates somewhat).

But if your plants look good compared to plants provided more frequent fresh nutrients, for whatever reason, I wouldn't get too worried about finding problems that are invisible. My plants always perk up with new nutrients, so I notice the positive effect, and I have plenty of uses for the discarded old nutrient solution so it is more as situation - yeah! got some great fertilizer for the soil plants and burnt out parts of the lawn.

That said, some professional outfits continuously back blend 25% and I suspect even higher amounts of the old nutrient when preparing new batches, so there is good reason to accuse a lot of manufacturers of promoting the overuse of their products so they can sell you more.