“How Long” Is the Wrong Question — “How Much Water” Is the Right One
A desiccant does not last for a fixed number of days; it lasts until it has absorbed its capacity of water. Whether that is one week or six months depends entirely on how much moisture it faces — the humidity, the temperature, the air volume and the hygroscopic packing around it. The same sachet that lasts a year in a sealed dry cabinet can saturate in days inside a hot, humid container. That is why desiccant is sized by the DIN 55474 method to the moisture load, not sold by a shelf-life number.
Two timeframes matter and are often confused. Shelf life is how long a desiccant stays good in its sealed packaging before use — typically around two years. Service life is how long it keeps working once exposed — from hours to months depending on conditions. Once a desiccant reaches capacity it stops protecting and, in the case of silica gel in rising heat, can even release some moisture back, which is why correct sizing and a humidity indicator card matter.