powersurge wrote:You cannot estimate your 12v battery by time. It depends on how it is stressed, and how well it keeps at a full charge.
I recently replaced an ORIGINAL EQUIPMENT battery that was 10 years old!!
The battery can be measured objectively, but it takes more than a simple voltage measurement. It must be placed under a substantial and calibrated load. Load testers can be had fairly cheap (around $20 for a 100-amp tester). This will tell you the general condition of the battery instead of just measuring the surface charge. ICE vehicles have a built-in "load test" of a sort, in the form of the starter motor. Sluggish starts generally let ICE drivers know when the end is near. With EVs it will sneak up on you unless you load-test periodically.
The other aspect is sudden failure, which can occur in flooded batteries as grid material is shed over time and accumulates on the bottom of the case. Batteries are designed with room at the bottom for this debris to collect, but if/when it gets deep enough to contact the plates that's the end of the battery. So that is an age-related failure mode that is more difficult to predict.