i have the 5inch rhino think it says ther good for 2 years of use, but all will depend on RH , more moisture less life !! mine is 6 months old i will smell the air that comes out of the duct sometime soon and then keep my eye on it from then on till it wont clean the air anymore.
if i did get 2 years that would be incredible value as they retail depending on size 6 inch £ 50/60 for 2 years of smell protection would break down to less than a dollar a week for piece of mind,, thats if i get 2 years mind, lol,, we shall see !