Canon Rebel T2i Quick Start Manual page 21

Other Image Sizes and Frame rates:
When recording 1920 x 1080
video, the Canon Rebel cameras
can only record 12 minutes of
continuous video.
If you require a longer recording
without interruption, you may
shoot for 24 minutes with the 640
x 480 option. The image quality
will be lower and fine image
details may be lost. The image will
also be more square shaped (4.3
instead of 16.9 aspect ratio).
1280 x 720 also records only for 12
minutes continuously because it is
recording at a higher frame rate.