6 volt batteries, discharge limits

For maximum life recharge at the 50% mark which I remember as 12.1 V. You should be able to look them up online Using Google search or the like.

Open ended question. Do you want maximum battery life? Do you want to use maximum amp hours? Fifty percent isn’t a magic number. This isn’t simple while some RVers would like it to be. It’s a number on a chart. Here is Trojan’s chart...

Maximum battery life is not achieved by discharging to only fifty percent. According to Trojan’s chart, maximum charted life is obtained by a twenty percent discharge maximum. Twenty percent discharges equal 3000 cycles. Fifty percent discharges equal 1200 cycles.

Most people will tell you 50% discharge, but my first pair of 6 volt batteries I discharged until the lights got dim because I did not know any better and I still got 7 years out of them. 50% is a very safe number but don't freak out if you drop below that.

The deeper you discharge the shorter the life. The shallower you discharge the longer the life. It's a curve as that above posted Trojan chart demonstrates. I would look up the discharge/life curve for your batteries and pick your compromise point.

If you cycle them down 80% 50 times a year they should last more than 10 years according to the chart.
Bigger issue IMO is to get them charged back tip top 100% often enough that they do not sulphate.