Moore's Law states, in paraphrase, that processing power as a whole doubles every eighteen months. Sometimes it's been clock speed, sometimes it's been number of transistors, but this seems to have generally held true.
The obvious corollary is that newer processors --computers in general-- are more powerful than older ones. This has consistently been born out, with extreme cases being the emulation of entire old systems on newer ones, even many at once, even without being emulated on the newest systems. (See: Linux to Game Developers: No More Excuses)
Based on this, I've thought up a potentially interesting, but probably wrong, corollary: Every eighteen months, it becomes possible to add another layer of emulation, without effectively slowing down the deepest layer. This means that, for example, I can have my MacBook Air emulating a Mac from a year and a half ago, emulating a three-year-old Mac, emulating a Mac eighteen months older than that, all the way down to the original Macintosh -- with the deepest Mac having an effectively indistinguishable user-experience (for better or worse).
Practically speaking, assuming an arbitrary six-month delay before full emulation of a system, this means that twenty-four months after a system comes out, it's possible to emulate it indistinguishably.
Of course, this is all armchair garbage. I have no numbers of any sort to back it up. I have no intention of going looking for them, although you're welcome to throw any you find/have at me if you want :)