Olympia, it depends on how bad your tank is. Purigen is supposed to take out ammonia, nitrite, nitrate, phosphate, silicates, and like a host of other things. In my 29gal, it lasted one week before it needed recharging but that tank has issues. The water is always yellow and I don't know why. In your goldy tank . . . who knows?
Seriously, does anyone know how much 3 5" dojo loaches, a 3" peppered loach, 1 stunted swordtail girl, and 5 congo tetras pee? I took out so many snails and yet the water is still yellow. I'd say there are maybe 30 snails left in there . . .