That depends on your strain. If you pick a strain from www.sensiseeds.com it clearly states how long till flowering( so the 12/12 cycle ) and it shows maximum yield. If you do use a 400-1000watt HPS, I suggest that you purchase a 1000watt HPS/MH Switcher for only 260$ US. http://cgi.ebay.com/ws/eBayISAPI.dll...PageName=WD10V 269.96$ For an HPS/MH Switcher with a panel and chains to hang it all. Its a good deal because then you dont have to purchase 2 1000watt lights. Good luck with your grow .