How long an electric boat motor runs comes down to one simple sum: your battery capacity divided by your power draw. Capacity is measured in watt-hours (Wh), and a kilowatt-hour (kWh) is just 1,000 Wh.
A 1.2 kWh battery holds 1,200 Wh. If your motor draws 200 watts at a gentle cruising speed, that is 1,200 รท 200 = about 6 hours of running. Push harder to 300 watts and it drops to around 4 hours. Open the throttle fully and you might pull ten times that power, cutting the run to well under an hour.
So there is no single figure. Runtime is something you control with the throttle. The slower you go, the longer you run, and the relationship is steep because drag rises sharply with speed.
To see how runtime turns into distance on the water, read how far an electric boat can go, or model your own boat with the range calculator.