i thought that bout one day for the plants to notice, but seems the hormones start to change as soon plant is exposed to set time of 'night' so if u flip 12/12 ur plant will notice in that 24 hours, takes bout a 5/7 days for visual, changing the time of 12/12 when already on 12/12 by having anothe 12 hours light will make it think its going back to veg thus stressing it, wont really make a massive affect on growth/yield but less stress is best:jointsmile: if u need to change the times.. do it the dark