In order to keep the database a sane size, it might be a good idea to update the added column when a user is seen and already in the database. Then a script can be ran externally to irssi to delete records older than N days.
I've done this in my fork in the following commit, but since I've tweaked a bunch of other little things I'm not quite sure how to do a pull request.
Honestly this is a bit of an XY problem. I'm stating a solution but didn't state the problem...
I've finding irssi freezing up on me every time stalker does something in the database. I'm going to go ahead and log a separate issue about that.
I wasn't sure what else to do besides clean up old records. Also, it's not a terrible thing to have the ability to do anyway. I'd personally maybe make it one year vs three months like I've defaulted to in my script, if needed at all.