Depends on the quality of the battery, there are batteries that are warrantied to last for 5 years, and there are ones that only lasts 2 years. Ultimately when they cant hold a charge or hold amperage, then it can not do its purpose anymore and its time to replace it.
Only a amperage load tester can check this, not a volt meter.
It depends on location and climate. In Florida, you are lucky to get 2 years out of one. In Iowa, my father had a factory battery last 13 years.
So hot and humid, start watching it at 2 years. It would be good to have it tested yearly anyways. You will notice a weak start when they start to go funny and a cell dies in it.
Read this post on battery life
When a battery *load test says the stored amperage is too low. Amperage spins starter motors not voltage.
When the engine doesn't spin fast in cold weather on start up.
When you start up your car and it takes longer than normal
when totally can't charge..