Sadly there is no way to calculate the recording length as we are using a compression algorithm.
The length will depend on the number of channels you will use but also how each channel will change.
If you display a static color it will use way less memory than if you are changing the color value at every frame.
For exemple also, moving heads are using more memory as most of time the pan and tilt values are changing.
Because all of that, it's really complicated to get an idea of how much you will be able to record.
We did some test and with 4 full universes of pixels changing colors all the time, we have been able to record 3 minutes on a Quadcore (that means in the worst scenario), we did the same test with a CueCore3 and we reached around 40 minutes.
Now, if you only have 320 ch, you should be able to record around 12 minutes of RGB pixels. So if the track you plan to record is not really dynamic with movement and color changing, it should fit without any problem in a cuecore or quadcore.
But note that in this case you will be able to create only one track in the cuecore/quadcore. AS they do not use dynamic memory assignment, if you do 2 tracks, you will have 2 tracks of 6 minutes.
With the Cuecore 3 we integrated the dynamic memory assignment, that means you can make one track of 40 minutes and other tracks of a few seconds.
I hope this will help you,
Enjoy