Only way to know how long a battery will last is to give it a try. A lot has to do with the motor and battery. If you’ve got the room, a second battery is the way to go.

As for electric downriggers, my opinion is they are a luxury, not a necessity. They’re not any more effective, just less of a workout. I regularly crank 12 pound pancake weights Mack fishing 100’+ in slightly more than a minute. My 10 year old daughter can do it in a little over 2 minutes (yes that’s me taking a jab at my fellow geezers). But, I’m also cheap and like to limit the number of things that can go wrong. Lots of stories about bad plugs, blown fuses, motor failures, broken drive belts etc. All that said a buddy has them and it is indeed nice to just hit the switch.