It's not possible with standard firmware, not ideal with Magic Lantern either. From the ML wikipage:
How do I record for more than 12 minutes?
Lower the bitrate_ (CBR 0.4 will let you record continuously for 30 minutes).
Use Movie restart_, but you will lose a few seconds when a new file is created.
To record continuously for more than 30 minutes, you need to use a HDMI recorder. Enable the Clear Overlays feature to hide the focus box and the 16:9 bars, and make the half-shutter button sticky to prevent the camera from turning off LiveView after 30 minutes.
Technically, there's no 12 minute limit. There's a 30 minute limit and a 4 GB limit, whichever comes first. With default bitrate settings, the 4 GB limit is reached after around 12 minutes (more or less).
The limit isn't just the 4GB FAT32 filesize, it's also due to import taxes to the EU.