Heat is the most serious aggravating condition to shorten battery life. It speeds up the chemical reaction that puts an insulating lead sulfate coating on the plates. Disuse is neck-in-neck for troublemaking, letting a "slime" coating form on the plates.
Here in Phoenix batteries are lucky to last 3 to 5 years. I usually get a 60 month battery and end up having a replacement prorated out around 40 to 50 months.
I did have one episode of great comedy with a Benz battery though...A parts monkey tried to pick up the old battery when I exchanged for a new one. He wasn't expecting it to be so heavy. His upper body moved, The Benz battery didn't. He darn near kissed the thing.
Scott