How many days will pass before orchids react at very low level of light?
 

For example at 50 or 60 % less light respect what are the need of a specific variety?

What will happen immediately is slower growth and a lack of flowering. I don't know how long it would take you to notice...

Think of the light input as you would to a solar cell - light is the plant's driving force, after all - less light exposure = less energy production.

For example, for an easy visual feedback, on Cymbidium i read that the leaves change color from light green to dark green but i don't known if:
this happen for all types of orchids;
how many days is necessary;
if the answer at a lack of light (on term of numbers of days) is the same between all varieties.

The visible signs such as the example of the Cymbidium, does not necessarily occur in all orchids, particularly the shadier growing types.

It's difficult to tell. Each orchid is different.

In some cases, you'll never know until the plant dies.