It isn't only cosmic rays. Regular old radiation can also cause it. I've read about a server that had many repeated problems and the techs replaced the entire motherboard at one point.

Then one of them brought in his personal Geiger counter and found the radiation coming off the steel in that rack case was significantly higher than background.

You may never know when the metal you use was recycled from something used to hold radioactive materials.

In the mean time I read here the rate to be around 1 bit flip per 1 GB per month. So in an 120 GB System that shd be about 1 flip every 6h. But then this number might be wrong.