Hey CraptastiC - Welcome to the site.
I'd say Electric, 100% - You're concerns with the gas are spot on IMO.
As far as how long batteries last, depends entirely on the set-up and environmental variables.
1) How many lbs-thrust of a trolling motor
Normally relative to boat size, for my 16.5' bass tracker my 60 lb-thrust gets me around pretty good, smaller boats could get by fine with a 36 lb as long as it isn't too windy, etc. lots of other sizes and efficiencies out there to explore as well.
2) 12v or 24v trolling motor, how many batteries, quality of batteries, etc.
I have a 24 volt trolling motor that requires 2 12v deep cycle batteries hooked up in "series" to complete the 24 volt circuit. I used to have a 12 volt motor where I would also use 2 12v deep cycle batteries hooked up in "parallel" essentially doubling battery capacity, and fishing time.
Batteries come in different sizes and amp hours, etc. Manufacturer plays a role such as Optima's technology for example but come with a much higher price tag. All things to consider.
3) Wind on any given day
Sometimes in order troll at crappie speeds (+ or - 1MPH) my foot pedal is on power 2/10, others 3.5/10 and so on. Windy days drain batteries. Many crappie fishermen don't like to wind for various reasons.
In the short of it, my batteries could last as long as 2 days of fishing without a charge or only 1 day if you add a fair amount of wind. I do my best to recharge the batteries at the end of day of fishing no matter what. Hope this helps, I'd be happy to answer any other specific questions. Most folks on this site run similar set-ups to mine, but everyone varies to a degree.
If you look closely in my avatar, I was "in-between" bow mount trolling motors for about a year when one took a crap, I am using my old minn kota stern mount 12v, 36 lb-thrust on my boat that was my 8th grade graduation gift in 2001 - although it wasn't nearly as convenient of a set-up, the latest and greatest isn't required for trolling crappies.