The life expectancy of a MacBook battery is mostly based on the battery cycle count. Once the MacBook battery cycle count has reached the limit set out by Apple (between 300 - 1000), the battery is considered to be at the end of it's life.

As the battery gets cycled, it's performance gradually gets worse. Once the limit set by the manufacturer is reached, you'll notice that the battery depletes rapidly. At this stage, you'll typically only be able to get 1 - 2 hours out of your battery life.

To check your MacBook's cycle count, go to:

Once you've completed the above steps, you will see the following window:

As you use your Mac notebook battery, charge cycles are accumulated (unless you use BatteryBox). A charge cycle means a full discharge of your battery from 100% to 0%. If you use 50% of your battery one day, then charge it fully, you've only used half a charge cycle. In this example, a full charge cycle will occur once you've again discharged your battery another 50%.

ie. if you use only 50% of your battery, you've only used 1/2 of a charge cycle.

Once you've reached your device's charge cycle limit, it's recommended you replace your battery.