Email Downloading Frequency?

Is there a setting that determines how often the phone sync's to get email?  I have a Gmail account, and it seems like those pop into the phone soon after I receive them on another computer.  I have a few other POP3 accounts, where it takes minutes or even an hour or two before I am alerted that there is new email.  I can tap the SYNC button, and that will retrieve them, but I am looking for a setting that makes it more automatic and immediate.

Thanks in advance!

H.