Hi there,
It depends on many factors, genetics, soil moisture level, temperature, watering frequency, oxygen level. But I must say, the roots love to spread downwards and laterally in a short time. Think of a 3 liter pot. If the conditions are optimal, you can see roots coming from below in a couple of weeks. And as they grow, their nutrition need increases.
Layered soil sounds off though. I'd let the whole root system access the nutrients evenly.
Good luck with your grow.