At what point is it more efficient to leave a light on when leaving and the returning to a room?
Since turning an incandescent bulb on and off doesn’t shorten the life of its filament significantly, you do well to turn it off whenever possible. The same isn’t true of a fluorescent tube—turning it on ages its filaments significantly (due to sputtering processes) so you shouldn’t turn a fluorescent lamp off if you plan to restart it in less than about 1 minute.