I have ask Susan about it but I really haven't received a good answer. I know the list is cleared completely off and I have seen it clear in the morning, noonish and in the evening. It looks like it's a manual operation but if so, I haven't found it in the manual. There is a setting that limits the size of the list but it's an administrator setting that's not visible to me. Without administrator access, I have taken it as far as I can.
There is a reason for limiting the size of the list. Each member has a copy of the list and with the number of users we have, each entry in the list would take at least 120,000 bytes of storage. With the current size of memory, that actually isn't that much memory. More likely a linked list approach is used and that would up the memory requirements to about 500,000 bytes per list entry. This makes it desirable to maintain a reasonable list size to avoid eating up all the processor memory.
If a copy of this list is retained on permanent storage, a fair amount of hard drive bandwidth would be required to take periodic backups of this information. This could interfere with the performance of the site for the up to 1000 users who could be on the site at any given time. At that is one of the reasons why Susan needs your donations to maintain the site and upgrade the hardware as needed.