Hi,
Ampere hour capacity varies from battery to battery, even of the same make and model. When new it should deliver the stated capacity and the test loads will show up on the data sheet for that battery. For a given load, you should be able to find out how long you can run that load to get down to 50 percent capacity. The time will vary with load and it is not linear at all but follows an exponential law, but looking at the data sheet they should show different capacities at different load currents.
As the battery ages however, the capacity does down. That means if when new you could load it for 32 hours with 1 amp to get to 50 percent, you may have to cut that back to 25 hours for example when it is a couple years old. You may be able to get some idea what to expect from the data sheet by looking at the capacity with aging graph if they provide one. If not, you may have to just wing it.
While the battery is powering something you can integrate the current over time to see how much capacity has been used so far. For a constant current this is easy. Once you establish an operating set, you could probably go by that for the future use of the battery. In other words, get the 50 percent capacity voltage from the data sheet and time how long it takes to get to that voltage with your given load. Each time you recharge, start the timing procedure over again, so if it took say 20 hours to reach 12.2v when new and that was 50 percent capacity, then note how long it takes to get to that same voltage each time the battery is used. If it gets down to 12.2v in only 10 hours after several uses, you know the battery is getting old.
The key here though is the data sheet, for the first measurements. After that it's just a matter of timing the discharge to the same point each time it is used.
I think the only truly reliable method for estimating charge without lots of battery data is to measure the specific gravity of the electrolyte. That means you need an open cell top battery. The relationship between specific gravity and state of charge is very accurate.