Meaning of a sentence using the perfect continuous tense
They basically mean the same thing, but:
A- Both could mean that the action is finished at the time of utterance's production.
Present Perfect Continuous has 2 uses:
- The action started in the past and has stopped recently or just stopped and the results are visible now. (You're late! What have you been doing?)
- The action started in the past and is continuing now. (We've been working a lot. It's time to stop for a cup of tea.)
B- Only the second one says when the action started being explicit about it.
(The examples provided are taken from my personal grammar, so you know they're correct.)
The first one more likely means that they've stopped now. The second implies that they are still throwing papers.
Compare it with rain:
It has been raining here.
It has been raining here since last week.
The second makes me feel that it's still raining; the first doesn't.