I know that in English, there's no perfect or pure future, I mean, I have to use the "will" plus a verb to talk about something that will occur. But my question is: when and how do you know if this "future" will come? I mean, if I want to say that something will take several times to happen I can say that: "They will marry after a long time?"

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With your example, you would say: "Eventually, they will (they'll) get married" or "they will get married eventually"

You could also say things like:
In a few years, I'll (I will) graduate.

When I finish college, I'll get a job.

Before I take the test, I'll do a lot of studying. (In this case, it is certain that you will study) But, you could also say:
Before I take the test, I need to study. (Here, you're just saying that you should study before the test)