After the growth period you have to take out your metal halide lamp and install a high pressure sodium lamp. You also have to change the light cycle from 18 to 12 hours per day. Flowering time varies depending on the type of weed you grow ,indica usually takes 8 to 10 weeks whereas sativa can take anything up to 16 weeks from when you start the 12 hour light cycle 'till the buds are ready to pick.