Depends on the plant cycle but for example on my 1000w mars hydro on seedlings stage I keep them around 24 inches from plant. As the plant grows I lower it though it’s stages. This also depends on the light you are using. Best accurate way to know is using a PAR Meter and measure the PPM’s per cycles.
Don’t think it too much, have fun!, and research! 🌱💚🤘🏼