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Leinad,
I use a Canon 20Da.
As Gary indicated it'll stay open pretty much as long as its powered but as Leon says, you'll have to experiment to see what becomes an acceptable level of thermal noise.
I have pushed my exposures to 20 and I think once to 30 minutes.
I have found that the best I get out of my camera, which has been a similar experience of a friend who uses a 350D is to aim for shots at ISO 800 between 8 min to 12 minutes.
I regularly take 15 min but if I can I try to aim for 8 to 12, seems to get me the best results without generating too much noise.
Incidentally, I use the plug in adapter instead of batteries as this is suppose to generate a bit less of the dreaded "burn mark" ni the lower right hand corner of the image.
I also no longer tend to shoot darks at all unless i really feel like it and have time to kill. Likewise, I never use ICNR, never (i di at first then stopped) If you are going to use dark frames, you're better off getting more light frames and then some darks.
The reason i dont use darks or dont worry as much about them anymore (i have a few master dark frames saved in a file I can refer t oif i really want to) is because of the great litle fature in DeepSkyStacker called cosmetics. Set it at 1 pix and 1% and it magically removes all noise without affecting the resolution/quality (trust me I'v conducted truckloads of tests on it)
Anyways,
hope this helps,
summary:
consider the power adapter for the Canon
use ISO800
aim for 8 to 12 min shots for faint DSOs (obviously less for very bright objects-seconds for the core of Orion M42 for example, a minute for globular clusters, etc.).
Get the free DeepSkyStacker to stack frames
most importantly, experiment and HAVE FUN!
That's what its all about!
cheers
frank
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