during flowering stages, towards the end, but people say u can do it through-out its flowering stage.. 1 tablespoon per gallon of water then bump it to 2 tablespoons.. *just what someone posted when i asked today*
I have the same brand, but the dark version with a green label, its unsulphered too. I use 1tspn/gal every other feeding since she was 2 weeks old. Most people use it to flush with during the last 2 weeks of flowering so your plant is still getting fed when nutes stop.