My girls have been vegging 24/7 for about 2 months and are about 24" tall. I'm using miracle grow (yes i know its bad) moisture control potting soil. I have an assortment of nutes i bought from advance nutrients but i'm not sure what i should be using... or how much for that matter. I'm afraid of using nutes honestly....
Well your using MG soil which is pre nuted so good luck on figureing out how much you need...thats why well one of the reasons why its highly not suggested to use that soil. Anyhow I use Advanced nutes its easy. Just read the back and use that as a max reference. Start out 25% or 50% of what it says see how it goes. I believe it says 4 ml per liter. Do the math converisons if you dont use metric. Easy.
In MG you can begin to use small amounts of nutes about week 3-4 IF you are using one pot throughout the entire grow(not transplanting). My friend uses MG soil and he first used MG bottled nutes with the soil and had an ok grow, he then switched to Flora Formula Nute line-up and had a much better grow. All he told me was he began nutes half strength around end of veg and went full strength by week 2 of flower. As rhap said, go by the directions and start out weak and work your way up. But for Pete's sake, flush the crap out of that soil a full 2 weeks before harvest. Peace