Dry camping - battery usage - very simple question

[hippieforever3]

I'm still struggling with trying to figure out if my batteries are
trashed or somehow I have an inordinately high current drain.

The facts;
4 - 12volt - 85ah marine deep cycle batteries in parallel
Batteries have been fully charged w/ external charger to 14.5+volts
current drain with almost everything off (sleep condition) = 6amps
measured with a good quality clamp ammeter

Question
How long should healthy batteries be able to maintain cranking voltage?

Regards,
GPSGary