Dark reaction of gum coating
I tested this once and found that within 5 hours, the gum layer was entirely hardened. I posted the result here several years ago and found it still on my website. Thanks for the reminder, because this needs to be linked into my technical pages; at the moment it's sitting there an orphan page and I'd forgotten about it entirely.
Because I live in a fairly humid environment, I don't keep coated paper around for this reason. But others in different environments may not find it such a serious problem. The variables I've identified as associated with the speed of dark reaction are humidity, heat, and dichromate concentration, but there may be more.