I'm assuming for the 90g you have in your signature? One cup is the max you should ever need, and you only add more if they dissolve or escape the reactor. I like to turn mine off, measure the amount in there with a piece of tape (typically marking the date on the tape, as well), then check from time to time.
If you run these, please be sure not to run GFO, UV, or O3. If you are using any of those, slow down on them as you should need them with what a biopellet reactor is designed to do (add beneficial bacteria to reduce nitrates).
I don't run mine right now, but I've had great success with them. I will be using one as soon as my system matures a bit more. I still run GFO when I need it, as well as O3 and UV (cautiously).