How do you know that's about the future?

Because we use the progressive to express the planned future. A: Tell Alex if he wants the key, he can come and get it from me here at the library. B: Well, I’ll give you his number in case you go somewhere. A: I’m not going anywhere. I’m sitting here for five hour cramming for an exam.

So that can't be about a situation in progress at the moment of speaking as in a phone conversation:
(А) Что делаешь?
(Б) Сижу здесь пять часов, занимаюсь.?
I've been sitting here for five hours...¿

It can be in progress, or it can not be in progress yet, but in either case, the speaker WILL BE there for five hours.

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