And as of yet, no one has mentioned items vanishing for good. All of mine are there and I check them periodically to make sure they haven't changed for some reason.
Ched posted this interesting tidbit in another thread:
Don't check hourly. You could check monthly if you wanted to - they stick around forever just waiting to be found. Checking hourly, daily or monthly makes no difference. You'll find the same amount of items.
If that is indeed correct then it might possibly indicate that item spawns do not happen when you're searching the map itself (which is what I'd been suspecting for a while).
That's what it's seemed like. It's one of the first things people tested for when they spawned cheap items. But it's good to know that there's no decay point as now I don't feel rushed to pick up the Cesium-137 on my screen for fear of losing it forever.
The only question that remains then is how many items can the map support at once. And the answer I'm suspecting is that the map can hold however many items there are spawn points for and I suspect that there are quite a lot. I'll edit in a screenshot in a bit to show you why I think this. There could be a cut off point, but there's no evidence to suggest this yet.
And after a day of zero spawns, I just got two more. Another Axe and a Can of Red Cow. Total simultaneous on screen items is now forty (40).
This is probably my most item dense section of the map. But the part that I found interesting are the two items directly left of the scrolling arrows. At one click zoomed out, they're practically on top of each other. Other sections of my map have clusters of items very close to one another. My best guess is that there are many, many spawning points.