I made two exposures of the sky in my front yard, one for 30 min, and one for 100 min.
General rule for long time exposure is to set your iso at lowest possible settings, with an aperture of f8 or smaller. A neutral density filter would help darken the exposure even further for extremely long exposures. A mechanical film camera is the best for this kind of application. If you have to use a digital camera, you must make sure the camera’s sensor noise is within controlable level so it does not interference with your final image. It is always a good practice to turn the camera’s noise control function on to reduce electron noise caused by the sensor. Also, most digital camera’s batteries will not last longer than 2-3 hours on bulb setting. Shoot in raw, so it will be easier for you later to reduce/enhance the exposure and control the color cast.