You are looking for the mean time between failure (mtbf) and mean time to failure (mttf).
Both of these are dependent of the quality (defects are possible) of the RAM as well as its failure rate. The failure rate is mainly dependent on the total device hours and an acceleration factor, at which the cells fail. Other parameters include temperature, uptime and energy...
A document that goes into detail on this is Hybrid Memory Products Ltd - SRAM Module - MTBF analysis. In this example, the used memory lasts over a lifetime for hundreds of years. You can see various memory manufacturers report the same thing, here is an example from Kingston:
Our process works so well that our mean time between failure rating exceeds 500 years!
The gist of this is that ECC is there to cover up for hardware mistakes or extreme usage, this is the reason why you often see it installed in servers as they don't want to risk having faulty memory.
From the other question, there is a study on this that shows different results, 50 - 167 errors per month rather than an error after a long lifetime. Now who speaks the truth? Did Google properly use MemTest?
Google has come out swinging on this issue. See http://blogs.zdnet.com/storage/?p=638 for how this really does affect modern-day systems.
This however is from 2009 based on data from the years before that, so it might be different these days.