Quote Originally Posted by deftdrummer
better rule of thumb: change when it needs to be changed! How do you know when it needs to be changed?

-is it starting to smell funky? say perhaps like a stagnant pond
- is there significant buildup on the side of the res chamber?
- do you see chunks or a film like layer at all on the top of the water?
- has the color gone from light or semi transparent to murky and dark?
- does pH fluctuate wildly from day to day?

If you have 1 or any number of these symptoms then it is probably a good idea to change the nutrients. I think it is always better to change early than change late or chance it at all. Your plants will thank you. Sorry if effort is not what you want to put into it but if ease of growing was your intent then soil would have been a better choice cause hydro is high maintenance.
Sorry to disagree deftdrummer but that does not hold true for most plants.

You are not counting for the nutrients the plant needs for the given cycle of life the plant is in at that time. There are only (X) number of available nutrients in the water. Once they are used up there is no more. When you ad back nutrients what do you ad?

N? P? K?

What has the plant used and what is left behind? What is a third order mineral (that is minerals discarded by the plant that combine with other minerals to make a third compound and even a forth compound. 99% of the time they are in such small amounts the plant can easy disregard them. But if you don't change you nutrients on a timly basis then they compound and will become a problem (commonly referred to as toxic salt build up)

Yes a rule of thumb is one thing but when it comes to hydroponics and the cycle of life for the plant then follow the plant not the calendar.