I've been running it in a water filter housing (BRS reactor) on my 66g for about a year now. I use one cup of the regular capacity GFO. Flow through the reactor is set very slow, it's just barely "bubbling/churning" on the surface of the GFO in the reactor. Early on I would test weekly to see if I needed to change it out, now I just watch the glass and it lasts from 3-5 weeks.
Don't go too fast with it, dropping po4 too fast is not good.