How much moisture can silica gel adsorb before it's saturated? This can be easily measured when drying it, by weighing it before and after drying.
Typical silica gel, usually used as desiccant:
wet mass = 91 g -> becomes 72 g when dry. saturated = 26% water + 74% dry SiO2 (by weight).
Cat litter silica gel:
wet mass = 54 -> becomes 47 when dry. saturated = 15% water + 85% dry SiO2
So, cat litter silica gel is about twice less efficient by weight than the proper stuff. However, it is also about twice less dense overall, so it takes about 4 times as much cat litter by volume than proper silica gel to adsorb given amount of water.
So, the ultimate figure: To adsorb 1 g of water, I need 6.7 grams of dry cat litter.
My crude silica gel dryer is made from a heater of an electric kettle, thermocouple to control the temperature and a phase regulator to adjust power. On top, I've put a few layers of aluminium foil, which is to provide thermal insulation and infrared reflectivity to help heat the silica gel inside more evenly. A hole in the top is to let the steam out.
It is ridiculously unsafe, with no overheating protection and exposed mains wiring. But, it allows me to easily test if the gel has finished drying or not, by placing something cold over the hole. If I see condensation - the gel isn't dry yet.