12v SLA charging, when do I have to stop?
If you want it fully charged, you need to charge it to 14.4 V and wait until the current has dropped to almost nothing. This CV phase of the charging is called absorption and is what puts it from 80 % state of charge to 100 %. If you have no means of measuring the current, you can estimate the time needed to the same as it took from 0-80 %. If you leave it connected to 14.4 V for extended time, you will slowly outgas the water as hydrogen and oxygen. For a flooded battery, just refill it with destilled water. For a sealed battery, any hydrogen and oxygen which does not recombine within the battery is lost forever and performance will suffer permanently.

After you disconnect the charger, the battery will fall quickly to 13 V and if you wait even longer, you will see 12.6 V. It's still fully charged.

I could not find a picture online with state of charge plotted along with voltage and current so you just have to take my word for it. Here is a 24 V battery so just divide voltages by 2 for your 12 V battery: