How often does a snake shed its skin?

I know the snakes have to shed their skins when they reach a certain size.

How often do they shed skins?
Do they stop shed skins when they reach full adult size? Since the shed of the skin is needed to grow bigger, when they no longer grow bigger, it should stop, right?

Well, they will always shed. It becomes less frequent as the snake ages. Young snakes of most species shed every 4-6 weeks. It also depends on how much they are eating, thus how fast they are growing. Adults of many species will only shed every 3 - 6 months. And most snakes never completely stop growing. Many slow to an extent that it is not noticeable.

The giant constrictors naturally have indeterminate growth, meaning they never stop growing. The rock pythons, Indian pythons, reticulated pythons, green anacondas, possibly amethystine & scrub pythons, and possibly king cobras are in this category. All snakes that are healthy will shed their whole lives, albeit much less frequently once they reach their adult size.

Never stop shedding and growing?
Does that mean the older the snakes, the bigger they are?

If some snakes live an unusual long live, wouldn't it become a giant snake we see in some horror snake movies?

My 10-year old ball python still sheds about once every 2 months. Factors that affect it besides growth rate include humidity, diet, etc.