The instruction manual.
I was going to comment, but felt its better for an elation rep to let them know which is right rather then my opinion.
It says that then lens should be replaced every two years, but really it only needs to be replaced when it gets cloudy or breaks IMO. The manual is referring to the outer condensing lens, by the way, the one you can touch (but shouldn't) on the front of the head.
Overall, you shouldn't need to replace any lens' on your power spot until it breaks.