Do you want to know......50% power as measured in watts?
Do you want to know......50% power as seen by the human eye?
You have asked about time. Time on, Time off, over span of time. ????
(Time on = 1mS, Time off = 1mS) and this happened for 1 week of time, then you will send 1/2 as much power out in light compared to 100% on for the same week. (on,off,on,off,on,off,on,off,on,off,on,off,on,off,on,off,-------for 1 week) 50% of a week, 50% of a day, 50% of a second is 50%.