According to them around week 8 is the time for a dose of 6ml/gallon, which is too strong imo. I used it at 1ml/gallon and burned the crap out of one of my nute sensitive pants. Use it carefully as it's strong.
Edit: Forgot to mention, Canna says to only use it once, not with every feed. I've used it 3 times now and I think that's why one of my girls was affected negatively..