I've randomly decided to not empty my spam filter, as I am wont to do on a roughly daily basis. Some back-of-the-envelope calculations (as well as the numbers on Abby's account) suggest that I can look forward to around 1500 or more before my spam folder starts to auto-delete messages. Of course, this has less to do with the volume I can accumulate within a 30 day span and more to do with how high that number gets before I can't take it any more and delete the lot.
Right now, I'm at 93.
Make that 94.