How large is a 20 minute movie from the Nikon D800/D800E?
The largest possible file size that the D800 can generate is 4GB due to limitations of the file system of the memory card. In practice, a 20 minute 1920x1080p full HD movie at 30fps in high quality is about 3.5GB (gigabytes). This may seem small but is made possible by h.264 B-Frame compression encoding. You can't change compression quality other than the 'normal' or 'high' quality setting. According to reviews, the high quality setting seems to be pretty good in terms of image quality.
If the camera stops recording before the card is full or the 20 minutes are reached, it's likely that the sustained minimum writing speed of your memory card is too slow. Another cause may be the overheating of the camera sensor. Let the camera cool down before you continue shooting.
If you need uncompressed recording, use an external recorder. The benefit of these recording is that you can make all sorts of corrections (color, contrast, sharpness) on the uncompressed signal. This is interesting for professionals; if you don't make corrections in post the quality difference is negligible.
Just a further comment. The same thing happened to me. the D800 would only record for 20 minutes and produced a file size just under 4gig. The card I used was a 64gb Sandisk 60mb/s. I thought this should have kept recording for around an hour at least. Any suggestions that would enable a recording of an hour using the D800??