From what I've read 30(min) - 50(max)w/sq ft (Google)
Got a Lux meter?
From Royal Queen Seeds -
Clones and seedlings: 5,000–7,000 lux
Vegetative growth: 15,000–50,000 lux
Flowering: 45,000–65,000 lux
Maximum recommended amount of light: 75,000 lux
Good luck