Eventually you'll have to add nutes. When to start depends on the richness of the soil you are using. I'd watch for yellowing of lower fan leaves before starting extra feed.
In pots of soil/soilless I don't check ppm but go by mls/L. My AN 3-part says 4mls/L but I will use 1 or 2/L and rarely 3 much less 4.
The plants kind of tell me when they want more or I just guessimate it from experience.
In DWC I start clones at 300 and add more nutes as they grow and rarely change nutes.