Being that type of guy, I tucked a bag of silica gel desiccant and a humidity indicator card into a #10 can of baking powder, then recorded the bag weight whenever I refilled the kitchen container:
For reasons not relevant here, we pretty much stopped using baking powder a couple of years ago, so there’s a protracted silence between the last two data points:
That last point emptied the can and, after a few days in the 60% RH basement, the bag weighed 243 g. The slope of the line suggests it’s been near 240 g for a while, although the humidity card showed the air was reasonably dry in there.
At our current rate, we’ll open the next can in a year or so …
The Smell of Molten Projects in the Morning 