GFO media will become exhausted over time, it can only hold so much phosphate per dose of media. If you have elevated levels of phosphate, it will exhaust much quicker. The exact length of time GFO will last in your tank will vary based on how much phosphate is being added into your aquarium on a regular basis.
In order to monitor the GFO, simply test your aquariums phosphate levels regularly. When phosphate levels begin to rise, swap out your GFO with a new batch.
In the case of elevated phosphate levels (more than 1.0 ppm), do not double up the amount of GFO you're using, rather swap the media out with a new dose more often. When your phosphate tests reveal the levels have stopped dropping, it's time for a new dose.
You can usually get back into the desired phosphate range with that second dose of media, then simply monitor the levels and look for a rise in phosphate levels.
GFO is best applied via a fluidized media reactor which increases the contact time with the media and also prevents settling and channeling. If you try to use GFO in a media bag, the water will only come in contact with the outer layer of media resulting in poor performance.