How quickly does GFO burn out?

[Randy Holmes-Farley Verified Community Expert]

How about this ... running 2 little fishes phosban reactor and had GFO in there ... normal amount nothing crazy. Was running a Maxi Jet 900 pump and was working great. After about a week, it turned into a solid lump and the water would not even move it to where it was tumbling -- like it was doing for a good week and half. Once I removed the old GFO media and put new stuff in there, it started tumbling again and was good to go.

Is this an indication that it may have been exhausted?

Not really. It is an indication that calcium carbonate has precipitated onto it, which is a drawback of GFO and which also greatly reduces it effectiveness and it is time to replace it, but it was not necessarily exhausted by phosphate binding.