I have had images up to 20 sec, without much noise problem.
However, I think that I did activate the long exposure noise reduction in firmware. I don't have the ZD with me and won't have access to it until Friday. I'll confirm the exact setting name by then but I would think that the manual has information on this, doesn't it?
Most DSLR also need a dark frame substraction to have noise free long exposures, the ZD is not any different. It is probably not as good as the Nikon, Canon and Phase, but the Dalsa sensor is known for not performing as well as the Kodak sensor in this area.