First grow, no$$ ... advice for improvement?

Alright this is my first time growing a plant of any kind lol...but basically all I could afford was a 100W HPS bulb, some soil, and some like no name 15.15.15 nutrients. Its a totally open setup in a large room, the temperature stays basically room temperature plus a bit of heat from the light. Never really gets too hot tho because the room is open. I water it once every two days usually, sometimes the leaves will start sagging a bit but after watering they stiffen up. Im not exactly how long its been growing, maybe 2 weeks or so. I can only have it for another 3 months top, im just wondering how big i should let it get before i switch the lighting to 12/12. (Its 18/6 right now).

with only a 100 watt hps i wouldn't let it grow too big because if it grows to tall the light won't be able to get to the bottom of the plant, wat i would do is bush the plant out and flower when it is about a foot to 15 inches tall and make sure if your watering every other day that your not feeding it every watering or you will have a lot of problems in the future

o ya and grow about 5 plants, there is a very good chance that you plant will die and if the plant does die you will be shit outta luck and have to start all over but if you have a few than your odds are better and there is still the chance that if will be a MALE....

to be sure you will probably want to switch in 2 weeks or so. You wouldnt want to get o the end of the 3 months and they arent ready..and 8-10 weeks isnt uncommon for flowering period..sometimes longer!