The amount of amp hours required varies by boat usage. If you're a marina hopper, you need little, plugging into shorepower regularly. Many of us enjoy boating on the hook, which requires three things: an understanding of how much load will be placed on the storage house battery bank, the size of the house bank, and the many different ways in which to recharge the house bank. (Some add the need for a battery amp-hour monitor, easily one of the best and simplest methods of "doing the math.") Generally, recharging comes from: shorepower, alternator from the engine, portable generators, and/or solar.
Attached is an Energy Budget form I've used for many years. Please note that it includes three types of sailing: daysails, overnights and sailing overnight, which are three vastly different energy consumers. I've added calculations at the bottom to project house bank sizes and have kept current with the actual battery banks we have onboard.
There is a photo of our Link 2000 showing a daily load of 100 ah here: Breaking in New Wet Cell Batteries: http://c34.org/bbs/index.php/topic,6353.0.htmlJust click on the pdf link below and use the Open button.
I started with a spreadsheet and copied it into a pdf for posting here. The math's easy to do to make your own spreadsheet.
For further discussion about what COMES OUT of the banks and what you NEED to PUT BACK IN, see the discussion of BATTERY ACCEPTANCE, here: http://c34.org/bbs/index.php/topic,4787.0.html