For the best nutrients, try this search: nutrients site:boards.cannabis.com - Google

And yes, you are correct about the 3 to 4 weeks. That's usually how long it takes for the plants to initially use up the nutrients in the soil. But they will need less nutrients when younger, more when older. So even if you re-pot during flowering, it's still possible to add nutrients. The dosage depends on the soil used and the plants' needs (seedlings from the same strain may have different needs), so it's something you learn from experience.

Happy growing :jointsmile: