There are rare times the atmosphere settles down. What usually happens with planetary imaging is that perhaps 50% of image frames are used in the final image. With deep sky images generally, you must use longer exposures so the images are always slightly blurred and so you use most of them.
With the next generation or 2 of camera sensor's, it may be possible to nearly eliminate read noise so we will be able to use very short exposures. Already there are such devices that called EMCCD but they are very expensive.