J C wrote:

I'm debating about whether to buy one.
What is the longest useful exposure one can use at room
temperature(around
20 C)?

Canon 10D: 4921 seconds is my maximum. You can see the results at
www.photo.net; click on the "Dark noise calibration" article, and
scroll down to the comment section for a 4x4 grid of (underexposed)
images.

I did the experiment when someone in rec.photo.digital was blabbering
about how horrible digital sensors were at long-exposure photography.
I _EXPECTED_ crap.

I was _amazed_ at what I got.