Magic Lantern is an alternate boot firmware (open-source) and is available for quite a number of Canon bodies.
The firmware isn't flashed into the camera... it actually boots and runs out of the camera memory card. It's possible to flag a memory card as "bootable" on a Canon. The camera has to do this. You use your computer to copy the Magic Lantern firmware to your memory card. You put the card into the camera and go into the camera's firmware update menu and behave as if you want to flash the firmware. What this REALLY does is run's some code which marks the memory card as bootable.
If the card is in the camera when it's powered on, then it'll run Magic Lantern (from the card). If a different card is in the camera when it's powered on then it'll run the normal Canon firmware.
VERY IMPORTANT: Magic Lantern isn't "complete" firmware. It alters menus and adds quite a bit of functionality. But for most things that it does, it invokes code that really is inside the camera in Canon's firmware. This means that the real Canon firmware must be in your camera and must be the EXACT version of Canon firmware that Magic Lantern expects to find. Magic Lantern (to my knowledge) does not "test" the in-camera firmware to make sure you have the right version. So read the documentation at the Magic Lantern website CAREFULLY.
The 7D port of Magic Lantern is currently considered "alpha" code (meaning you should expect bugs). If your camera hangs you must EJECT THE BATTERY from the camera to fix it. Do not merely power off the camera. The power switch does not physically cut power to the camera. It merely tells the computer that you'd like to power off. The initiates a power off cycle (e.g. tells the lens to "park" the IS elements, tells the sensor to do a "dust" cleaning cycle, etc.) and then finally puts the camera into a powered off state where it's monitoring the power switch for a command to wake everything back up. But if the firmware has "hung" the computer, it wont respond to a power-off command. That's why you have to eject the battery (that does cut power.)
The 7D version of Magic Lantern says that it wants you to have Canon firmware version 2.0.3 installed in the camera. If you do not have 2.0.3 (and it MUST be that EXACT version... not 2.0.2... not 2.0.4... it must be 2.0.3 (why do I feel like this sounds like Monte Python and the Holy Grail in the scene with the Holy Hand Grenade?)
And the Lord spake, saying, 'First shalt thou take out the Holy Pin. Then, shalt thou count to three, no more, no less. Three shalt be the number thou shalt count, and the number of the counting shall be three. Four shalt thou not count, nor either count thou two, excepting that thou then proceed to three. Five is right out. Once the number three, being the third number, be reached, then lobbest thou thy Holy Hand Grenade of Antioch towards thy foe, who, being naughty in my sight, shall snuff it.'
Anyway, you get the idea. You're running alpha code. It has bugs. It can hang your camera. The firmware version must be precisely what they want.
The functionality you want is called an "intervalometer". It can be programmed to repeatedly take photos at any specific interval for any specific number of shots or duration. Canon has a remote release that has an intervalometer on it. Magic Lantern has a built-in intervalometer that runs in the firmware.
