Capturing more than a few minutes with a single exposure is impossible on digital cameras and increasingly difficult and error prone using film. To circumvent that, I use dozens or hundreds of single exposures and merge them with software to compute a digitally captured long exposure photograph.
I take pictures in fixed timeslots, exactly like you would do to capture images for a timelapse video. How many depends on the subject and the time window which should be captured, but usually never more than 2 or 3 per minute.
Step 1: to handle the RAWs I prefer Lightroom, just set the white balance and export 16bit TIFFs in full size.
Step 2: the images may need to be aligned, depending on the sturdiness of the tripod and other factors. If the camera is mounted for a very long time, even the tiniest amount of movement adds up.
Step 3: merging.
The whole process can be seen in the example video:
Left: single image, Right: long exposure
While I work on the software, new images are compiled with new versions and may yield different characteristics.
By the way: if you want to know something in particular about an image, have a look at the EXIFs. The compressor software does now - and will be doing to an even greater extent sometime in the future - write everything important in the image metadata.
The code can be found on github.