With that consideration I present the following (derived from an original in the Dragon Kings high-level play sourcebook for the only true Dark Sun setting):
|Time Elapsed||Paper||Wood||Metal||Soft St.*||Hard Stone**|
* Limestone, sandstone, marble, tufa, etc.
** Granite, flint, mica schist, nephrite jade, gritstone, etc.
The table shows the average effects of time and the elements on various materials. This will vary be prevailing climate (hot, wet and windy will accelerate erosion, cold, dry and windless retard it). Items protected from the elements will take longer to erode. Multiply erosion time by ~10 if in a sheltered location, and by more if items have either been properly curated, or sealed in a preservative anaerobic environment).
Paper and wooden objects fade over time, making it difficult, but not impossible to read or identify surface features. Fragile objects must survive an item saving throw versus fall every time they are used; brittle items must survive versus a crushing blow. Crumbled items are unusable but still identifiable as papyrus or wood; dust is completely unidentifiable and unusable.
Metal and stone items wear away over time, their shapes smoothing out until the item is completely worn away. Paint indicates that artificial coloration is gone or unrecognisable; etching means carved letters or pictures are worn away; relief indicates that deeply carved letters or images are severely eroded; form indicates that time has eroded away all but the basic form of the original stone or metal.
Why am I bothering with this? Well, there's an ongoing subplot involving Hobgoblins (think a timelost version of Japanese holdout soldiers + some Tekumel flavouring) and a particular time-locked sub-level of the Vaults...