it will depend on where the phosphates are coming from, how long it has been a problem, size of tank, amount of GFO you run, etc...
Please post more information like tank size, feeding amount, system equipment, amount of live rock etc...
For example: I have been fixing a phosphate problem for 6 months in my tank, a 90 gallon tank with 30 gallon sump that has 120lbs of live rock. The tank had high level of phosphates for a long time so the rock absorbed the phosphates and are releasing them now. I could have used other methods to speed up the process but I chose a long slow process since I did not want to take my rock out and "cook" them. I am using GFO & carbon dosing to lower my phosphates slowly.