I use it for houseplants in the winter, and use it outside in my garden if spring/summer/fall but wouldn’t use it for other hydro. To much hassle to get it right again but usually fine for the other plants if in soil as a buffer. 
My 4 site with exterior res uses 81-120L depending so I wanted to save on nutes as well but found that every 2 weeks is the sweet spot for me to have a total res change. 3 weeks depending on when in the cycle can work but when i’ve had problems it’s ALWAYS near the 3 week mark.
Best of successes!