It depends on your internet connection speed!!
I have tried on 1MB internet speed and I could rip 1Min 35sec!! exactly KSV is right, cause during 2 mins you can't download more than 7MB ~ 1min 35sec video (if you rip 7MB of this stream, you will see your video has 1min 35sec duration), with 1MB internet connection speed!!!
Try
StreamTransport!! It gives much better result in compare rtmpdump!!