How long can the new trends go before need to change out

My seedlings have gone 10 days with its first nut the PPM is around 700 my pH balance is between 5.7 and 6 and it's still good do I need to change it now or how long can I wait I'm using the complete kit General Hydroponics nutrients

General rule of thumb is one week but I have gone longer many times. That PPM sounds pretty high for seedlings... What's your base water ppm?