Can someone please explain the 12 minute limitation better... All I want to know is if when the 12 minute clip is up... can you record another clip immediately following? If it's a SD card size limitation, can you just swap cards and keep shooting? If its a sensor overheating issue, how long does it take the sensor to cool down?
when the 12 minute clip is up... can you record another clip immediately following?
Yes. You can literally hit record again and away it goes. BUT, you have to be watching the camera to notice that it's stopped.
If it's a SD card size limitation, can you just swap cards and keep shooting?
The limitation is an individual file size limitation of the file system used on these cameras (FAT32*.) A file can't be larger than 4GB, but you can store multiple 4GB files on the same SD card. So you can just hit record again - without having to switch cards - until the card if full.
If its a sensor overheating issue, how long does it take the sensor to cool down?
It's not clear that the 12 minute limitation is there as any precaution against overheating; you can actually record for 29 minutes and 59 seconds in SD mode.
That said, depending upon the ambient temperature, and whether you are recording in 1080p or 720p, the temp warning seems to appear anyway between half an hour and two hours of use. Your mileage may vary. I think whether you're hand holding or using a rig or tripod, how much time is shooting vs. non shooting, also will effect your results.
Realistically, you're looking at 10-15 minutes to make any meaningful difference in internal temperature. But some people report a camera shutting down, leaving it off for 5 minutes, and then recording again.
Though many users have reported getting the warning while shooting for extended periods, they are often able to keep going with the warning flashing without noticeable ill effects to the footage.
You might try some kind of additional (external) cooling. Shutting off the camera while not in use is probably a good idea too.
A second camera may be the best solution if you are going to be doing a lot of non-stop shooting, and literally can't afford to wait for the camera to cool down.
* The T2i also works with SDXC cards, which support exFAT, but it seems to have the same 4GB limitation.