He has been on holiday for two months = by definition, he is still on holiday. At the time of my speaking, continues to be on holiday.
By definition means: up to or at the time of speaking. That is really what defines the present perfect. Not stopping or not stopping an activity. It does not inform us about that really.
I've been reading this book for two hours.=And will continue
OR: will stop doing so but the present perfect does not specifically tell us that at all.
Compare: He was on holiday for two months.
He no longer is on holiday.
Just try to not overthink it. That's just the way it is.
- You can't complain because you have been on holiday for 2 months! [at the time of speaking, two months have elapsed. A different meaning than the first one.]
Compare: You can't complain because you were on holiday for 2 months!
That's enough I think for one question; the continuous is another whole thing.
By the way, this usage rule applies to all uses of the present prefect.
Visual representation of the present perfect:
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