Stainless steel has an inherent rust and corrosion protection although this happens out of the box, so time and exposure to outdoor elements for sure create these processes in the metal, causing in some circumstances the total failure of the piece after a while. That way, some would say that no type of production from humanity could withstand outdoor exposure during a period, say one hundred thousand years. Is it true?
Diamonds base their exceptional strength in the crystallization of carbon. They are a carbon allotrope like graphite and then graphene or carbon nanotubes. That reminds us how carbon is one of the greatest key elements for its amazing possibilities, and role in the life. Other curiosity is how those elements can be created naturally with ease.
The original question remains unanswered and perhaps it doesn't have a general answer because there are too many factors involved for causing the complete degradation of the diamonds, affecting its presumed and impossible eternal condition. Diamonds are crystals so they have intrinsic irregularities that affect their resistance to breakage, but erosion doesn't place like in any other. Diamond fusion point is a bit above 3800K, lava is considerably less so they can survive on this. The diamonds that arrive on the surface could reach billion years, after more than a hassle. If those estimations are true, that could imply an important percentage of Earth's age! So practically forever.