You don't have to as long as you don't care if it is effective. After a month or two it will reduce the ability to affect the water. I use it to remove tannin released from my wood so I have a crystal clear tank.
I have read that you can reconstitute it by placing it in a high salt content water solution. If interested, you can Google it. I replace mine every couple months and the tank is noticeably more clear compared to the "ice tea" looking water that the wood resulted in.