ROWAphos is extremely effective. It will start working immediately. However, you may not see the results in the first instance. If this is the first time you have used a phosphate remover, or you have allowed phosphate to build up in your tank, then repeated replacement of media may be required. I have experienced this myself. I allowed my phosphate to climb to 0.1ppm to see if it has any effect on my coral colour. In the process, phosphate became bound to my rocks. When I turned the reactor back on again and tested for phosphate, it remained at the higher reading. I had to replace the ROWAphos every three days before I noticed a reduction.
Once phosphate has been drawn from the rocks, and is stable between 0.02ppm and 0.05ppm (the ideal range to keep algae away), then you should only need to replace ROWAphos every 4 to 6 weeks. I aim four every 4 weeks. Test regularly to find your happy medium, and replace the ROWAphos as required for your tank.