I am new to LEDs myself. I am using 300W in total. 120W + 180W. As far as watering, the best advice would be as above, to watch your soil and your plant. I can say that with LEDs it is a little different when it comes to noticing small changes in the growing environment, coz everything is pink/purple!
It kinda screws with your eyes don't you think? Put up some pics if you can.