There's to much emphasis on these batteries. Hook em up and keep em charged. And love the sound it can produce. When they don't work anymore, buy some new ones and start over. I'm glad I don't think about my car battery as much as these audio batteries. I couldn't go anywhere.
Yes, it should be straightforward but it's also pretty easy to drastically reduce battery lifespan by letting them discharge too much.
If, after 3 or 4 months the batteries no longer are able to power the same load for the originial 12 hours.......
If this has happened it's a simple indication that the batteries have been allowed to discharge too deeply, too often. If you can keep your discharges to no lower than 70% [of full charge] battery life will be greatly extended, as shown on cynan's graph. What you can also see, is that extension of life is nearly exponential, so if you can limit the discharge to 80%, you'll get significantly better results than 70%.
Bigger batteries = less % discharge for a given amount of use, so you should be able to choose a battery size to suit your usage.
I'm using a 25 year old set of ex-Telco batteries to power my house but I very rarely go below 85% discharge and they are in use every day of the week.