My experience with lead acid batteries is very similar as previously mentioned. I forgot twice to disconnect the battery in one of my cars and the battery was useless/destroyed after one year. On the other hand, a car I bought new it survived for more than ten years. In regular use (every few days) it should last at least 5 years.
Sometimes I do a battery capacity measurement (i.e. full charge then discharge with constant 5 Amps down to 10.8 Volt and measure the Amp*Hours).
Lead Acid degrade most, AGM are only marginally better, LiFePo4 are best. After 5 years, lead acid have lost half of their capacity, but are still good enough to start the engine if not longer cranking is required.
When buying a new battery, buy one from the big dealers, they have more throughput and batteries don't spend life on the shelfs...(and mark the date on it).
If you rarely use your car, you can install a battery cut-off/protection switch. This device disconnects the consumers from the battery to protect the battery from deep discharge which really kills a battery.
