Firstly the laptop DC-DC converter,
19.5v/12v = 1.625 increase in voltage, so 4.62A x 1.625 = 7.5A current draw.
Lets say its 80% efficient.
7.5 / 0.8 = 9.4A or 12x9.4 = 113 Watts
113+2.5+13 = 128.5 Watts total
128.5/12 = 10.8A MAXIMUM current draw.
The laptop is not likely to be running at max power tho, so most of the time I reckon you'd be somewhere between 5 to 7.5A draw.
If you have two 26Ah batteries in parallel (52Ah), and lets say a worst case of 10A draw, the batteries would supply 52Ah/10A = 5.2h....
But, SLA battery capacities are rated at 20 hours because it looks better, (26Ah/20h = 1.3A per hour for 20 hours), at higher currents there is loss due to internal resistance in the battery.
Jaycar says the 26Ah batteries are rated at 3.74A over 5h(18.7Ah), so two in parallel would handle ~7.5A for 5 hours.
There ya go, about 5 hours with two 26Ah batteries.
