To avoid deep-cycling, you should only use 25% of a car battery's capacity.  [1]

An average car battery is 40 amp-hours.
        {  25% of that is 10 amp-hours  }

My CPAP machine uses an average of 2.2 amps. [2]
        (the wall-plug PSU delivers 6.6 amps)

        >> TODO -- find out what the current draw at my settings is
        >> TODO -- turn off the heated hose and humidifier, and find out what the current draw is then

So if you want to avoid deep-cycling an average battery, my CPAP machine will run for 4.5 hours.
        {  10 amp-hours / 2.2 amps = 4.5 hours  }






[1] https://dashcamtalk.com/battery-discharge-prevention/#What_is_the_ideal_voltage_cut-off

[2] http://www.taleofatomato.com/2012/06/self-congratulation-post-2-cpap-power.html