it really depends. I use fox farm ocean forest in veg and i usually dont use nutes until after a month of veggin without nutes. however, each strain is different. some strains eat more than others. My wife had the mistake of vegging and then right before flowering transplanting the plant into a fresh patch of soil. She would then follow the instructions of the fox farm nutes which say feed them at first signs of flowering. So any way she would transplant them into a 3 gallon bag of fresh soil and after a week start the feeding process at 1/4 of the strenghth. i tell you, burned the plants to hell every time.
I say let the plants tell you when they are hungry. if you switch to flower and transplnted then in fresh soil right b4 the switch like my wife, let them use up all the nutes in the soil first b4 applying nutes. good luck