Well, I did dig up this paper, which claims that your RAM will get bit alterations from “Atmospheric Neutrons” (aka: Cosmic Rays) at a rate of about 1.3*10^-12 /bit/hour.
An article by Berke Durak uses that to calculate that your probablilty of having at least one bit error in 4 gigabytes of memory at sea level on planet Earth in 72 hours is over 95%. Of course that assumes you are using non-error-correcting memory (non-ECC). With ECC, he figured, you can wait 2.7 million years before you get an uncorrectable bit error at a probability of 96%.