Most but not all LA batteries will be damaged if taken below 12.5 volts for a long period, recharge as soon as you can.
The trickle charge value for a battery is dependent upon battery size, manufacturer, age and previous usage. The only good method is to adjust the trickle charge voltage so that the battery does not gain a charge, it just retains its charge, or even loses it very slowly. Recheck every 12 months of so and re-adjust if needed.
Charging above 13.6 volts will cause any LA battery to gas, even a sealed one! This also reduces total life expectancy of the battery.
When sizing a battery, assuming you want it to last a long time, buy one that 70% of the capacity will cover all your needs, then only charge to 13.4 volts (around 70% capacity), it will last a long, long time and never gas.
Battery gas can be dangerous to life and limb when it gets ignited by a stray spark or similar.....so its far better not to gas......humans also!!
For those who understand, no explanation is necessary, for those who don't, no explanation is possible!