Dried some silica gel. Put it into a box. And recorded weight of the box from day to day. After about half a month, I got enough data to find the moisture permeation rate.
0.52 grams of water per day (24h), at 55-45% humidity of outside air and 18-25°C temperature.
So, if I were to replace silica gel pack every month, I need a pack of 100 grams of cat litter. That's not too much, but replacing silica gel in every box (I have five so far) every month is not something I'd like to do.
Let's take a more reasonable target: replace each half-year (six months). Then, I need to put 600 grams of cat litter. That's half of that bag! There's no way I can fit that much into this box, because it's a rather tight box for a single spool!
BUT. I didn't seal up the lid with tape, and the lid isn't tight at all. So, the experiment will continue with the lid sealed up a bit by tape. Stay tuned, I haven't started this experiment yet.