I use 2 Thermapens in my restaurant daily. The older one that is about 5 years old still has the original batter, has been "washed" by the staff and generally semi-abused. I would say for someone not in the food service industry it should last a lifetime if you take care of it.
I forgot to add, I found my splash proof Thermapen floating in the dish washing sink the other day. Some how it got knocked into the water. I fished it out, opened the battery door to check for water. No water came out, so I put the batteries back in and kept on using it.