Equation to figure out battery bank

Guys,

Is there a simple formula to figure out how much battery you need in your bank?

Currently have (2) Interstate 24's
Running:
(1)JL Audio HD 600/4 (4- tower Bullet 7.7's)
(1)JL Audio XD 700/5 (4- boat Kicker 6.5's and 1-JL 12W3)

Thinking about changing to 1- JL 13W7 and HD1200/1 (instead of the sub channel on the 700/5)

I want to have enough to play 2 days with only the alternator topping them off and not deplete and kill the batteries (i.e. get more than 1 season out of them). I also don't want to have more than 3 batteries total, IF possible. I've always been the type to slightly overkill the situation, so I want to have some reserve still, even after 2 days of no shore charging.

I'm thinking of running (2) Group 31's for the battery bank and (1) group 27 for starting the boat. Am I going to need more? I understand the value of golf cart batteries, but really don't want to go there. I'd rather spend a bit more and keep it simple.

Thoughts?