In addition to the body, the lens that you use will make a difference - in particular the focus length and the aperture. Take a look at LonelySpec's article on the topic. He has a scoring criteria for comparing various lenses, based on their focus length, aperture and the sensor size along with the shutter speed...
Ian has reduced it to a spreadsheet...
You are going to be limited to the amount of shutter time, before the stars begin to start to trail. With Pentax using the GPS you can get up to 5 minutes on an image, but that will blur the landscape elements (since the sensor is tracking the stars, it blurs the items that are stationary). So, to keep everything in the frame in good focus, not blurred - you are going to be limited in terms of the shutter speed.
Pixel shift is affected by the Earth's rotation. Since the pixel shift takes 4 images in quick succession, if say 20 seconds is your maximum shutter time, then you would want to take 20 seconds/4 images equaling 5 seconds as the maximum shutter time for each individual pixel shifted shot.