The first situation I had with brittle plastic was when I opened up the airtight ammo box I had my first set of 1/72 (HO) figures in - Airfix WWI Doughboys - and they crumbled in my hands. This was over twenty years ago and twenty some-odd years after I bought them. The WWI French I bought at the same time are still in great shape. Some WWI German Infantry I bought shortly after those two boxes fell apart about ten years ago when I was using them for a project for my son's history class.
The worst case I had was with ESCI Zulus that crumbled just a few years after I bought them I got so disgusted I gave up on the era totally - particularly since they were all that was available for the Zulu War at the time.
I've had better fortune in 1/32 scale, I still have some of my very first toy soldiers - Marx ACW figures - I received for Christmas 1961. They are somewhat dirty from a multitude of battles in numerous backyards and sandboxes but except for an occasional missing flag or broken bayonet from battle damage are in pretty good shape.
I do know that since my Airfix tragedy I've taken to storing my figures in ziplock food storage bags in airtight boxlike containers as far out of sunlight as possible. I've noticed no further deterioration in plastic since.
As I mentioned before the airtight ammo boxes I used to store my guys in did not do much good. I had some figures actually get hit with mold that sped up the deterioration. Maybe they weren't so airtight?
So far the ziplock bags have been working well but only time will tell. It does put things in perspective. These beautifully sculpted and molded pieces of plastic are, after all, pieces of plastic - nothing lasts forever.