Ive always wondered that myself too. For a normal car battery, I am paranoid about draining it too much and causing permanent damage from sulfation so I make it a habit to try not to drain it past 80% SoC, which is 12.42v (with no load). But I have no idea how you would measure state of charge when the battery is being loaded. When loaded, I dont think the measured voltage will correlate to a specific state of charge because the voltage will vary depending on load.
If you are going to regularly be draining the battery past 80%, maybe you should get a deep cycle instead.