I've read that you could use it every time you do a WC. I've only used iy once, it was because of an anemone getting sucked into a wavemaker and I did 100% WC in stages. Once you have established bacteria in your system you should be ok.
I only use nitrifying bacteria only to start my tanks. Then I might just add it every couple of months to help keep the good bacteria. but in all honesty you only need to add it when you first start your system