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.