Wannabe, I kinda hestate to say much, but I've always run either 27
or 29 series Delco voyagers and seem to be getting much more run time
than that.
You can't really count what I was doing this wkend, pushing with the 8 horse
and steering with the tm - even tho the tm is set on 40-50 percent,
I don't think it is seeing the normal amp draw because the gas motor is
providing 99 percent of the speed, tm is just keeping up and steering.
Anyway, I can do that for two or three days if need be.
Now earlier this year, I pulled some in shallow water with tm only, from
1.5 to 1.8 mph and I know I fished 5-6 hours a few times and still had
two out of four leds lit up on the battery gage 
I know I've spider-rigged into the wind, running tm on 60-70 percent
just to hold still for similar times too.
Shoals Area Crappie Association