What needs to be changed on this model and how often

Nothing, for at least 5 years, if you clean it regularly. Which means at minimum, of every thirty days. Especially the air intake filter on the back and the plaques and any dust build up you see inside the unit. A can of Compressed Air with a straw nozzle (like you clean your computer with) works well for cleaning the dust out.

About 5 years down the road, you will probably have to replace the plagues (also called plates).

When it comes to air purifiers, it's like Benjamin Franklin said, "Cleanliness is next to Godliness."

Hope this info helped, Misty. Please let me know.