assuming your batteries are at full discharge, take the wall wart and see what output it delivers. I have a 396xt, its 800 ma output. 3 batteries, say 6900 ma so divide by the output of your charger , this is 8.6 +- hours. I safely add about 10% more and you would have 10 hours to go from a dead cell state to a full charge. Now I charge about 6 hours because I try to balance the time used and the replacement time to play again.
In general, higher capacity batteries last longer, but also take longer to charge. Major brand batteries are generally of good quality. You can also improve battery life by limiting/reducing the amount of time the backlight is on and using only the amount of speaker volume you need (turn down as far as you can). Backlight and audio amplifier are two of the biggest controllable users of power in the scanner.