I know, no-brainer, but what I mean is, they're not a regular scheduled replacement deal like oil/filter changes, etc.
Life depends on the battery and use. Cold weather climes can be tougher on them, shortening life.
As I said above, check the cable ends for corrosion first and clean them. If you have a multi-meter, check the battery voltage at the battery posts themselves. Should read 12.3-12.7 volts or so if fully charged.
If you get it started via jumper cables, check voltage again at the battery. Should read 14.3v or so at idle, that will let you know if the alternator is doing it's job.