64GB CF cards should fit more than 3 hours of 5D Mark II footage (44mbit/sec, or around 5.5MB/sec, which is variable. Roughly 12 minutes = 4GB of footage, inclusive of all HD frame rates). I've found that generally a LP-E6 battery lasts around 1-1.5 hours, so a 64GB card will require more battery life than a single E6 battery.
