Like in Steam where you know exactly when it ends.

Depends on the type of sale. Flash deals have a timer on the front page, and the page for weekend sales and the like tends to have a timer telling you when the sale ends. On the other hand, it's not unheard of for games to leave a big sale before it ends without any advance notice. So if something catches your eye, you might want to consider grabbing it sooner rather than later.

What's missing is a timer on the game page. You can see on the homepage how long a sales lasts, but it is a bit cumbersome to check all sales to see of which the game is part of.

If there is, I never found out. It's always annoying when I need to decide to purchase something and when I finally have spending money, the sale's over.

Discounts typically last at least two days if it isn't a misprice or insomnia

Don't trust the countdown timer, as they often pull discounts sooner and without any warning or notification

When there are overlapping promotions, the price can change

You get attuned to GOG's ways after a time