CHDK will allow you to set/program exposure times up to approximately 65 seconds (EXIF data will only list 15 seconds). It is impossible for the camera to take longer exposures.
As far as I know, there is no "Bulb" mode ("shutter" stays open while the button is held down). Only timed shutter releases.
There are a few threads here where people have taken multiple long exposures and added them together for a longer effective shutter time (apparently astronomers also do a similar trick).
Another issue you will need look at is Noise Reduction (actually, Dark Frame Subtraction). When the camera is set for 2 second or longer exposures, it first takes the "picture" and then again does an exposure but with the shutter closed.
The second exposure is subtracted from the first to remove "amp noise" and hot pixels. So your 65 second exposure takes about 130 seconds to complete.
CHDK allows you to turn NR to Auto, On, or off.... You can, for example take 5 "quick" exposures with NR off, and on the last shot, take a NR exposure and manual subtract them in your photo editing software.
That is about all I know... You can look around or search the threads here for more help on any details.
-Bill
