Connecting a CPAP machine to a battery?

I have used both a series one and a dreamstation. Both will run on 12v, with the proper cord, available from your supplier. I have a 12v outlet installed under the bed, which is connected to the 12v coach batteries. I hve run it all night several times, don't have any idea what the maximun time is, but probably can do at least two nights without recharge.