Any auto battery used monthly and competently charged in-between should last 4-8 years depending on conditions. This is halfway between a daily automotive use (where that is true) and less-than-once-yearly emergency-power use (where that is also true).
If the technology is lead-acid, age in years will be the single most important factor, and there will be little performance difference between a common run-of-the-mill Exide and a "quality" car battery, whatever that is. (Gold-plated cell interconnects? Corinthian leather plate spacers?)
Now it is possible to get other chemistries of battery such as nickel or lithium based. These require different charging curves and are not compatible with lead-acid chargers. For instance a 9-cell nickel pack will be destroyed if placed on a lead-acid charger; a 10-cell nickel pack will simply undercharge, which doesn't harm it. Lithium packs are even more feisty given their fiery failure modes; they absolutely require internal protective circuits to shield the battery from overcharge or undercharge.
Not every charger is equal
And the market is so flooded with cheap junk that it's really hard to find a good one. I suspect what happened is your staff bought what was readily available on the marketplace that claimed to be a "good one" (don't they all), slapped it on there, and expected the charger and the battery to "just get along". The charger was not a proper 3-stage charger, or didn't know how to shut off at the end of stage 3, and so it cheerfully overcharged the battery to its eventual doom.
This is so common that I assume this is what happened.
I gather no one was adding or checking water routinely; otherwise by month 3 they would be noticing the battery was using an awful lot of water and that would prompt them to take a hard look at the standby charger.
If this is an alt-technology battery like lithium, then it was either misapplied (9-cell nickel) or is a piece of junk.
If you want to leave a charger on 24x7, you need to use a charger capable of the task. Note this brand isn't even offered on Amazon; they mainly trade to industry, which has no tolerance for cheap junk that doesn't work.
If your car has battery-draining gremlins, fix them
You have to have a zero tolerance policy toward electrical problems that drain the battery when the key is in the "off" position. Your car should be able to sit 1 month uncharged and start right up. My car has sat 6 months uncharged and started (with a tiny bit of complaining) on a 3 year old battery.
The very fact you are putting the car on a charger for monthly gaps is itself a problem. When you allow a problem to fester, it has a domino effect of causing other problems - like you put it on a charger, the battery cooks out, you then drive it around with no functioning battery and burn something else out... The dominoes fall one-by-one.