"I have been working for six hours."
Translation:Pracowałam przez sześć godzin.
I would agree, but there is slightly different connotation I feel – "Pracowałem sześć godzin" feels like I could just as well spend 4 hours of those 6 checking out new lolcat memes, while "Pracowałem przez sześć godzin" feels a bit more like I actually did the work during those 6 hours – but that is very tiny difference and maybe it is even just my personal perception. :-)
No. That's mostly because Present Perfect translates poorly into Polish, it changes to simple past tense.