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.
Still dealing with this. Still showing Ammonium, shrimp are fine, still..and eating as usual, and being active. Nitrite spiked to .50 for some reason, today was WC change anyways on the EI schedule. So I did a solid 50% WC and all is well, except for the Ammonium reading. Not sure what else to do, I have tried adding "mulm" from my other established tanks..and just waiting it out. Also added some seachem stability to hopefully help.