Richard, put your boat in the water,tether it and run the motor flat out. Stick an ammeter in the battery line and measure the current. Divide the battery capacity, eg 1.5Ahr, by the motor current you just measured and you should wind up with a figure which will be the number of hours the boat should theoretically run until the battery is flat. For example, if the battery is 1.5Ahr and the motor draws 0.5A then in theory, the runtime will be 3hrs. Thats the theory but it assumes your battery is up to the claim made on the label. If its getting old and tired (like me) then it won,t stay the pace! Naturally, the bigger the battery, the longer the run time and the same measuring techniques can be applied.
Regards........Chris