Do steel objects have a maximum lifespan? I read that "316 stainless steel is estimated to last for 1200 years in a “rule“ environment before heavy pitting. In the marine environment this is reduced to a mere 260 years." Are they referring to steel exposed to the elements and not cared for? If a stainless object was periodically electroplated or shielded from the elements, would it remain otherwise stable and not degrade?
Strictly speaking, very few metals are "stable" in terms of the laws of thermodynamics. True chemical stability is when the atoms are in their lowest energy state. For most metallic elements, various oxides, sulfides, and chlorides are lower energy states than the pure or alloyed metal. This is why corrosion occurs in the first place--the atoms will tend to form compounds that reduce their overall free energy--iron becomes iron oxide, aluminum becomes aluminum oxide, etc.
Most alloys, including stainless steel, are 'metastable', meaning that they will--eventually--move toward a lower energy chemical state (i.e. they will eventually corrode). However the kinetics, or time-dependent processes of corrosion can be slowed down to the point that they appear stable over the normal human lifespan. As others have pointed out, various catalysts and corroding agents can accelerate this process. But even just sitting 'in a museum', degradation will proceed at some vanishingly slow rate.
So the short answer is, "yes, there is a lifespan for all alloys, but sometimes that lifespan is so long that it doesn't matter; and the service environment plays a huge role in how long that takes."
Edit: As an aside, this also means that some of the useful microstructures of steels (pearlite, austenite, martensite, etc.) will also tend to degrade over time since most of these are metastable. Even if the chemical identity of the material doesn't change, the atoms will gradually rearrange themselves over time to reach their most stable configuration. Again, this is usually (but not always) on time scales far beyond what you'll need to plan for. Nevertheless, this is one reason why you should always be aware of the temperature environment of your metals--even if the heated alloy doesn't outright corrode away, a few hundred degrees can drastically reduce the time required for microstructural degradation, which can dangerously weaken hardened steels.
Stainless steel has an average life expectancy of about 50-100 years. Look no further than the two most famous stainless steel monuments, the St. Louis arch and Chrysler building. While Chrysler is made of a high nickel/chrome alloy than most. The Arch is a great example, Upclose it's kinda dingy after 50 years. Inspections show it to be structurally sound but it's stainless veneer is rather ragged.