bash: /bin/rm: Argument list too long

I've noticed recently that rm has a limit on howmany files it can handle in a glob. Somewhere along the line some of my mailing list directories lost their expiration settings, and filled with 10s of thousands of messages. KMail was not completing the move to trash when given over an hour. I decided to delete the messages by hand, so I went into the ~/Mail directory and tried rm in the relevant subdirectories. That's when I saw "bash: /bin/rm: Argument list too long". I did for f in *; do rm $f; done, and that worked. I'm just curious. How many's too many? -- Regards, Steven