My first post. In Steam, the games in discount have a timer, that tell you how much time left you´ve to buy a game.
But here in GOG, I can not see this feature, can anybody explain me how much time discounted games last?

Most games discounted at any time are part of a sale. Gog has a weekly sale every week (funnily enough) that runs from Monday at 11am to Monday at 4pm (GMT), so usually that will be when a deal runs till.
There are exceptions, games that are on sale individually or other groups of games that are discounted but as far as I remember there is no way to tell when those discounts end if there hasn't been an official announcement.

There are of course other big sales through the year but you can tell from the front page when they are on.

When a discounted game is part of a bigger sale, then there is a timer on the sale's page. However if the game is discounted individually, as mentioned, there is no way to tell if it is not stated in accompanying official post by GOG.