Piece of string. Motor speed being a big one. I used to troll a bit for barra in a Canadian canoe with a 55lb riptide at half throttle and get a couple of hours out of an 85ah battery. Those numbers dropped quickly if I wanted to crank it up a bit. Short fat boats are generally more inefficient to push around due to displacement and waterline length calcs. I would think a couple of 100 ah's might get you a decent morning on the water with a bit up your sleeve to get home without rowing too much?