I wish ELM would only update the mailbox folder if there really is new mail. Oh, well, guess I have to wait for ELM4.0. However, I do not recommend this. This can take quite a while for big mailboxes. The check is then performed when you type CTRL-H in the index menu.
Some shells have a built-in facility to check folders for new mail.
They usually just check once in a while whether the timestamp of the files have changed. They do not look into the folder and look at the mails, though. The default is 60 seconds. When it is time to check for mail, the shell does so before prompting. If this variable is unset, the shell disables mail checking.