"She's been in London for five days".
I can't understand from this sentence, is she in London now?
It's ambiguous. You can't really know if she's still in London or not without further context. The verb tense, alone, doesn't determine this. All it relays definitively is that she has been in London for five days.
But speaking personally, if I heard only that sentence I would assume she had spent the last five days in London and was still there—even though I could be wrong.
However, those words don't necessitate either her current presence or absence from London. Nor do they specify how long ago she was in London—or even if those five days were from a single visit.
Here is a longer version of the sentence:
She's been in London, on and off, for the past five days.
Note how the added words change the interpretation.
Without the on and off, I would assume that the time she'd spent in London had been continuous. But with the on and off it now becomes unclear exactly how much time she has spent there.
With the addition of the past, it's no longer an uncertain time in the past, but an exact period of time in the past.
Different words will give different explicit meanings. Without those words, the situation is open to interpretation. (And because it's open to interpretation, different people will interpret it differently.) The only thing you can glean from the verb tense alone is that she has been in London (on one or more occasions) at some point in the past—and most likely for a total of five days.