What you you using to recompile the files into video? Virtual Dub?
For my part I import the DNG sequence into after effects, apply some color correction/white balance with camera RAW and then export a lossless video, which can be edited in Premiere.
I don't have a card fast enough to record at 720p at the moment, but to try to give you an answer (and bump)... I think a 32 GB card will hold ~13 minutes of RAW video. Please correct me if I'm wrong anyone out there running 720p RAW video.
At 1476x626 23.976 the RAW video bitrate stands around 35MB/s , so in a 32 GB Card we can theoretically record around 15 minutes of RAW.
EDIT : Magic lantern says the bitrate is around 35MB/s when recording but after verification the real bitrate is around 38 MB/s .
(RAW File 1476x626 for 273 frames@23.976fps = 430 249 KB)
So the recording time on a 32GB card is more like 14 minutes.