Sometimes car batteries can last up to 10 years, but more common is 4 to 6. Keep in mind that after the first few charge cycles, where your new battery actually gains a bit of capacity, after that it's all downhill. How steep and short that hill depends on how the battery is treated. In any case, it is losing it's capacity over time, and one day, will leave you in a lurch if not replaced ahead of time.
Signs of a weak battery are slow cranking, voltage dropping below about 10.5 volts during cranking etc. Low temperatures reduce a battery's output, while too high a temperature (think battery located under seat without added ventilation) reduce battery life.
The quality of the battery (e.g. made with virgin rather than recycled lead) has a lot to do with battery life as well as how it's treated *before* you bought it.