when to start flowering

plants are 4in tall but i can only grow for 7 more weeks in hydro,when should i start to flower them? i just want to some thing

not an expert of course but I've herd that they 8 weeks is a good veg time by veg I mean after you see your first 5 finger leaves,(also when I start nutes) thats where I am at I am wondering the same thing my self planted in soil mid feb