Jester: It's not necessary.
meppitech: That will depend upon the parameters of your source water and various other tank-specific parameters. Some members have Aquasoil that they've been using for years and years. On average, I've had mine last at least 3-4 years before there's any noticeable difference in buffering my RO/DI water in shrimp tanks. But that's with older products. The newer Amazonia seems to be much higher quality.
All that said, I can't imagine not having the itch to change up a tank's scape after 2-3 years. A year is about all I have in me until I want something new.