How long could you expect a depth finder to run if connected to a single battery as shown in the illustration?
You wouldn't want it connected like that as you'll get interference from the trolling motor. In the illustration, you are getting 24v but the same capacity so it would depend on how much the trolling motor was used. In overly simplified terms, think of just having a 12v trolling motor running off of one battery and then adding a FF to it would sort of answer your question. In reality the 24v trolling motor would be more efficient but you can see what I am saying.
If the batteries were connected in parallel, you'd be getting 12v but would double their capacity or amp hours. My cranking battery can't handle running lights at night for several hours and still have enough power to crank the motor. I plan on adding another battery in parallel to it or just put all of my lights and electronics on the extra battery as a stand alone.