I got 17 years out of the original battery in my L2800. Never on a maintainer and never needed a charge. Slowly losing power until I decided it was time to retire it. My BX1870 battery went 9 years. Same thing never maintainer or charged. One day it just didn't even have enough power to spin the engine.
I would think ambient temperature may be a factor in the life of a battery. Being as you are in a warmer climate your battery will not see the extreme below 0 temperatures my battery sees and the extra stress on the battery when starting a tractor with oil temps below 0.
I'll check back it I get 17 years on my tractor battery if it's still alive

I keep all my toys- motorcycles, snowmobiles, tractors, trailers on maintenance chargers all the time and get well over 15 years on all of them.
My neighbor has a 2008 JD garden tractor which I do maintenance on for her. She does not keep the thing on a maintenance battery charger as I recommend - even though I bought her one, and I have replaced the battery twice. I have checked the charging system on this tractor, and it is working fine.
This may mean nothing, I have no detailed facts on why, but all I know is that the stuff I keep on a maintenance charger full time keeps them alive.