Validity of any certificates in any language

I have always been asked this question by my students and everybody else. Here's the answer. And this applies to any certificate (Official Language School, Trinity College, Cambridge exams, TOEFL...). I hope you will understand it when you watch later the scene from the film Braveheart.

How long are any results and certificates valid for?

That is to say, like in the film Braveheart, "I was wondering if you could do this when it matters, as it matters in battle. (...) you could? Then prove it!..." See this scene and then you will understand why.