Battery Charge and Discharge time for constant voltage charge battery, and how to know if its fully charged
You're confused about charging times while you should not be. It is actually quite simple because you're using a SLA battery. You can charge such a battery simply until it reaches 13.8 V. This can be done simply by connecting it to a 13.8 V voltage source, which is your SMPS.

Do make sure there is some form of current limiting in place so that the current does not get out too large when the battery is empty and 11 V for example. I expect your SMPS to have proper current limiting but if it current limits at a high current, check that the battery is allowed to be charged with such a current. If not you may want to add a power resistor 
between the SMPS and the battery to limit the current.

When the battery is full it's voltage will be around 13.8 V and charging will stop automatically because battery and source will be 13.8 V. There is no need to keep track of charging time. Most Uninterruptible power supplies have SLA batteries and they charge them this way because it is simple and effective.