Do you test to determine when your GFO needs changing?

[christyf5]

I didn't know which to vote for as I test, I change on a regular basis or when my tank looks like I need it. I've slacked off on the testing as of late as I've been too busy (or too lazy) but for the most part I change it out every 2 months whether it needs it or not or when my tank has more algae than normal. I've also upped lately as I noticed that my tuxedo urchin had a serious Fabio hairdo of green hair algae. I also test every few weeks with the reading usually being 0.08-0.015ppm. The algae I grow doesn't seem to need much phosphate it would seem and only grows in areas where I can't see it (back glass) and the fish aren't interested in eating it (back glass).