when to change lights to 12/12

hey everyone. im growing in my closet and will be my first indoor grow. I got the jist of growing and need to kno when i shud change my timer to 12/12. Im growing feminized ice seeds in a closet under a 275 watt hps light. also if any1 knows any tips or advice to increase yeild please let me kno.. thanks guys hopin u can help

MJ plants will double or triple in height during the flowering cycle. The question of "when" to start the cycle depends upon the headroom of your growing space.

Yield is dependent upon a number of factors ... Amount of light, genetics and grower skill.

Vi