I don't use Rowaphos...it's too expensive. I use GFO from bulkreefsupply. It's the same stuff, but alot cheaper.
Anyway, for best results it should be used in a reactor with slow flow so the gfo tumbles inside it. Why are you using it? If it's for an algae problem, depending on how much you used, it may need to be changed every week or so.
I started using it about a year ago. I was starting to get macro algae sprouting up on some of the rock. Since I've had most of the rock in my tank 20 years, I figured they had to absorb some phosphate. About 2 months after I start using it, the macro disappeared.