They are hardy enough to be handled, but will chip if scratched, and break if dropped.
I store my crystals in a sealed container, and they are fine after a month. People who have left them outside do not report dehydration, but the surface still turns white due to exposure to light.
Some crystals can be successfully sealed in resin to prevent dehydration. Preventing light from photodecomposing potassium ferrioxalate crystals would be very tricky. I do not know whether it is possible.
Light reduces the iron center of the ferrioxalate ion, and oxidizes the oxalate ion into carbon dioxide. Since the green color of the solution is due to ferrioxalate ions, a loss of them will make the solution browner, which is the color of iron.
If there is excess oxalic acid present, then the extra oxalate ions can form ferrioxalate ions in the absence of light, causing the solution to revert to green.
Sorry about that. I tried to keep it short, as the guide is long enough already. Since some of you have commented about that, I'll make some edits to it. Thanks for the feedback.
I heated it at 200C for 1 hour on a hot plate. You might need to heat a bigger batch for longer. Generally, I do double the time for it to get completely dry, to make sure all the iron hydroxide is converted to iron oxide.