Hello.
According to the BA, every 12 months. There are also small adhesives, such as inspection stickers.
It is worth it.

Dear m.simon,
We recommend replacing the silver cube once a year before the season. This means that when you take the humidifier out of the cellar after the summer break, replace the silver cube and you are ready for the whole season. If you have any further questions, please do not hesitate to contact us by telephone: 041 720 48 48.
Kind regards,
Stadler Form

Thank you, Galaxus, thank you, Stadler Form.