This chart, shamelessly ripped from the Interwebs because the links keep rotting out, may prove useful in the future:
In round numbers: between 10% and 40%RH, silica gel equilibrates at 1.8%RH for each percent of weight gain. If you toss 100 g of dry silica gel into a container with some filament, when it weighs 120 g (20% weight gain) the air inside the container will be at about 36%RH.
The Smell of Molten Projects in the Morning 