If they are the type of codes that reset themselves after so many "key cycles" thereby extinguishing the CEL then they'll still be in there but if someone hooks up a scanner and erases them, they're gone.
They are definitely not in there forever. There is some limit to the memory in the PCM but I have no idea what it is. It's probably not that much though. Probably it's a FIFO buffer that would hold some reasonable number of codes for troubleshooting, maybe 8 or 16?