soy sauce is not as much in the fermentation time as it is in the colour. Light soy is lighter in colour and generally saltier than dark soy. Dark soy has additives such as caramel
to impart a reddish colour to your food (usually meat or tofu) when cooked.
Light soy sauce is in fact saltier than dark soy because it isn't "cut
" with additivies.
I should note that light is an unregulated word in the food industry, and can mean anything from low fat to light in colour.