For me..... 6-8 weeks, didn't cure my green issues but I can only assume it has improved my phos situation. 30 gal, brown barr goby and 2 chromis are the only inhabitants along with a prolific mixed reef.
it will eventually become ineffective, how soon depends on the amount used and the amount of phosphate present in the tank. I change mine every couple of months, when i'm running it that is....