With an 18 AH lithium battery and a good app like with the Impulse Lithium's you can kinda calculate how long it'll last. Here's screen shots of mine with just the livescope and a Garmin 106SV on it.
so 18 AH battery running just that will last 18 divided by 2.55, or just a hair over 7 hours. I find that it is pretty darn accurate in my experience. I love that Impulse App. It's free BTW.