Great question.
They are built last. They have an all steel body, powder coated finish an no complicated electronics to go bad. The manufacture gives you a 5 year warrenty on the air cleaner and the filter.
I can say from personal experience they can last a very long time. The Austin Air Jr in my office stopped working and it was 27 years old. We have 2 other units at the house and they are both over 15 years old.
You can read more about it - How Long Do Austin Air Healthmates Last