hps is not widely used as a vegative growth light as it doesnt hold the right colour spectrum(red) but it can be done with a hps so dont worry might jus take a lil longer! the timing for vegging should be 24/0 if you want to give the plants a break which i recomend! have 18hrs light 6 dark jus make sure your grow room has gd air circulation as it could burn the plants an stress them out somewhat!! when the plants reach an average of around 12" - 17" depending on strain you can switch the light cycle to 12/12 for flowering which the hps is great for! gd luck!