I used to get around 6 hrs or so when I ran MIC-IN, Phantom ON, Backlight ON, etc. So I would bet you will get around the same on your 744 since inputs 3/4 wont be used!
I used a 744T just once, for RAQ NYE Run 12/30/2007, and had a board patch for 3/4 and 483>744 for channels 1/2, and I recorded the whole show without worries, which was 2 sets long! And that was with a 6,600mah battery.
I also had made an external power solution w/ a Sony DK-415 722 battery dummy ce3ll, and used 7.2v/4,500mah RC Car batteries to power my 722 at festies/long shows/etc
And most importantly, have fun and pull some "heat"