i veg for about 2 n a half weeks or when the plants are about 1ft tall as the plants will triple insize and in my exsperience the white widow isnt a short plant.
also when you switch to 12/12 dont put the plants strait on flowering nutes carry on using veg for a week it seems to stop the stretching getting out of hand.
also as the plant is still growing and not truly flowering they develop better if given veg feed 1 week in bloom period.
also keep your lamp as close a possible to the plants as you can without burning them this also keeps things nice and compact with tighter nodes and less stretch.