You cannot beat the read noise of a camera. It is what it is by design. As a user you can make sure the read noise is the dominant noise contribution of your camera by identifying that exposure time where the Dark Signal Shot noise begins to exceed the Read Noise. That is one commonly accepted criteria for setting the maximum exposure time. The maximum time before "crossover" can be increased by additional cooling.
Another practical limit discussed in the literature is where the Dark Signal Fixed Pattern noise exceeds the Dark Signal Shot noise. Both are discussed in the passage below from Janesick.
If cooling of -15C is just sufficient to ensure the read noise is no greater than the dark shot noise for a 10 minute exposure, then for a 60 minute exposure the material below teaches that the dark current should be reduced by a factor of six to have the same noise profile in the camera. This speaks nothing about the amount of signal from the object and sky background you are capturing. You may or may not have sufficient signal after any given exposure time. That must be evaluated separately on an object by object basis considering sky conditions.
