It's really hard to place a time frame on battery life like weeks, months, or years. There are so many factors that go into determining the life of a battery it's hard to boil it down to how old the pack is. Some of the things that can determine the life of a battery are how many times it's been recharged, how it's been recharged (quick charge or slow charge), how the battery has been discharged, the temperature in the operating environment, vibration in operation, and many more factors go into determining how long the battery will last. The best way to determine if the pack is ready to be replaced is by its capacity. When you first get the battery cycle it to determine it's initial capacity (I usually to a 3 cycle set to get the capacity). Then throughout the life of the battery you should regularly cycle the battery (once a month or so) and compare the capacity to the initial capacity. When it drops down to 80% of the initial capacity I will replace that pack.

In your case, having to replace the pack after 2 years use is a completely reasonable time frame.

Ken