You should replace your Chemipure Elite when you notice you glass is getting that green haze on it.
As it looks like you are working on lowering your phosphates, you will have to change it more often than someone who is simply maintaining a low level, so you can't judge by asking how often people change their GFO type products.
Not known as overly accurate when testing phosphates.
See if you can find a local reefer to you who has a Hanna Meter or Checker to accurately test your phosphate levels so you know exactly where you are with them. I am sure they a quite a bit higher than an API kit is telling you
A couple of other points...it's important to know why you have high phosphates and do something about that - or you will be chasing these around until you quit the hobby
You may have too high a bioload for the tank, or possibly do not do enough or large enough water changes. You could have an under-performing skimmer, or none at all (which you would need to compensate for with more frequent/larger water changes)
Overfeeding can be an issue as well as there are phosphates in many of the foods we feed. Or you could be using tap water in your tank or have a ro/di that needs to have the filters changed.
You might have purchased rock that came from a tank with a phosphate issue and the phosphates could be leeching out of the rock...and so on & so on...
As you can see there can be many reasons why you might have high phosphates (there are others as well),, but these are all things you need to be aware.
Simply treating the symptom, will not get rid of the cause