I'm wondering what you're wanting to do that requires long exposures.
When I do astro-imaging, the shutter will stay open for several minutes. I have never hit a limit as to how long I can leave the shutter open... but there are reasons why I prefer not to leave the shutter open too long.
For example, when I take images of the night sky, even though the sky is "dark" there is still the problem of light pollution. After the shutter has been open long enough, everything starts to just look muddy because the black parts of the sky aren't so black. It would be highly unusual for me to take an image longer than 10 minutes.
A common reason that people want to leave shutters open for a long time is to take star-trails or star-lapse shots. But it turns out it works better to stack lots of shorter exposures (short being about 30 seconds each) because this is longe enough for stars to expose nicely but short enough to avoid collecting too much light from light pollution and turning the sky to mud.
This is why I said I'm wondering what you're wanting to do with long exposures... depending on your goal, there may be some other practical limits or issues and/or a different technique to achieve your desired result.
Long exposures will tend to drain the battery. When I do astro-imaging, I use the Canon AC adapter.
Tim Campbell
5D III, 5D IV, 60Da
