The knock off ones last about two weeks.
Not true - I bought a pair in Koh Phangan at Christmas 2012 (and wore them for the next 6 months pretty much straight) and they're still going strong. I shall persist with these until the ball of my foot works its way through to the earth.
That said, I did buy a knock-off pair of Converse flip-flops in Hanoi, and they quite literally fell apart within ten days.