>How long does a charge last?

It depends on how hungry your electronics are and how often you use them. Most depth finders don't draw too much current. I have used my 7.0AH battery for an entire 3 day weekend, on 1 charge, when using a depth finder only.
If you are using a bilge pump for recirculating water in your livewell then you should be aware of the current draw of your pump. A 500GPH uses less juice than a 750GPH pump etc.. I am currently using a Hummingbird Phiranah 5 depthfinder and an Attwood 750GPH bilge pump. If I use my depthfinder and I'm recirculating 3 times an hour for 5 to 8 minutes at a time my battery usually could use a charge after 1 full day. I also have a 12.0AH battery which lasts an entire 2 day tournament weekend no problem.
I think a 7.0AH battery is enough for most peoples needs and won't weigh you down too much.
MT