A lot depends on the growing medium and style that you are using. I grow in Promix and compost/manure and just use Jack's 20-20-20 after about 3-4 weeks, depending on size and if started from seed or clone. If you have pre fortified soil, like Fox Farms, then you probably won't need to feed them until flowering. I use tap water for minerals and don't really add anything else other than some wood ashes that I mixed into the Promix/manure mix. This just provides some buffering as well as major/minor nutrients.
As I said, I feed with Jack's 20-20-20, 1 tablespoon/gallon and feed about every 12 days or so. After you get to know your plants better, you will be able to judge when they are getting hungry just by their color and appearance. Good Luck!!