So the library's updated website indicates that you can "log in before clicking Add To My List to retain titles in the list for 90 days."
The 90 day limit was pretty upsetting, since I keep all my books to read in that list, and sometimes I don't read them for months. So I clicked the "Give us your feedback" link and gave it: while I understand that there are space constraints, that limit makes me sad.
Within the hour, I received a reply stating that the entire list is retained, and only deleted after 90 days of inactivity. I have no worries there--I'm in there all the time. Hooray!
Now, when I told this to Mike, he seemed to think the salient point of the story is that I'm someone who calls the library to ask how long it keeps my list. Now, I think that since they had a big BUTTON asking for my advice, there was nothing presumptuous about me offering it. But I'd like to know if I'm alone in thinking that there's nothing obsessive about this.