Can achieve half output with two V-mount or Gold Mount batteries (14.4V / 15A, 26V / 8.5A, 28.8V / 7.5A)
Can achieve max. power with two V-mount or Gold Mount batteries (26V / 18A, 28.8V / 16A)
Just divide that into 2 and then divide that Into the mAh of the battery. Assuming it's a fully charged in good condition battery. So 2 90Ah battery at 28.8v (28.8v means 2 fully charged 14.4v batteries, 26v batteries I'm assuming means 2 14.4v batteries that after usage the voltage has dropped to 13v) last at 50 % for 24hrs for and at 100% for 11.25hrs. U
You could use a multimeter to count it more accurately