Jennifer Brooks wrote:

You don't need to change it,LOL! Eheim wants you to change some because they
need you to buy more Same with the cylindars. Leave it right where it is
Jen
PS I LOVE my Ehiems too, best filters I have

What she said.

I will wash my substrate in a bucket of tankwater once a year or so.
This causes some of it to grind down into dust, which I discard.
Eventually I need to add a new handful to make up for the little
bit that gets lost, but that's it.

Jeff Dantzler