At reg 720 X 720 by 23 frames per sec., 4 gig will give you about 7hrs of record time depending on the detailed the photos are and the brightness of the video... the 8 gig will give twice that. most the time your battery will expire befor you can reach the 8 hr mark..
Well, the first thing you got to know about that is that it cuts your video while recording. In other words, once you start recording, it will record until it reaches at least 250MB and then it saves that part and starts automatically another part which will be saved after 500MB - 1.5GB. That is what it does till the power goes down. Normally, 1GB will record up to 50 minutes. What I am saying here is that, your pen will not record until it fills 4GB because a fully charged Pen doesn't record over 2hours. Any question?