Oil and water painting, maybe. On camera plastics, I certainly don't think. Never seen it happen.
Camera plastic and casing cracks and fade with heat/cold and sunlight. E.g leaving it inside baking car for instance which causes expansion and contraction. Then there is an issue between rubber lifespan "from usage" and lifespan "without usage" which has never been confirmed unless you put 2 similar unused rubber grip in various RH for determine lifespan. Too often, experience reported are dealing with used item which can also mean tears and perspiration damage.
A question that comes thru my mind is the lifespan of electronic component in electric drycab vs unopen airtight dryed box. Smaller airtight sealed box means limited oxygen and low moisture and reduce oxidation for decades. Does it mean electronic and metal parts will last longer (assuming all power batteries removed) with less oxidation.
How much to put will depend on how big your box is.
From my experience,
for my box - slightly bigger than the typical size of a tissue box,
i have a small plastic box full of small holes, (size about a match box)
i normally fill this 'matchbox' with only 1 layer of silica gel,
this will get the box to my targeted level of 45-50%.
I normally open/close the box in air-con room.
It should last for a month, with maybe 10 open/close.