Pretty sure most memory test tools like memtest86 write the memory and then read it back shortly thereafter in relatively small blocks. This makes the window for errors to be introduced dramatically smaller. Most memory in a computer is not being continually rewritten under normal use.
If you manage to replicate bitflips every few days your RAM is broken.
It's the "once every other year" type of bitflip that's the problem. The proverbial "cosmic ray" hitting your DRAM and flipping a bit. That will be caught by ECC but it'll most likely remain a total mystery if it causes your non-ECC hardware to crash.
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.