The newsletter is supposed to be sent out about once a month, or as often as big news happens. Usually, it's every three weeks or so. The goal is not to clog your in-box with bulk mail you could care less about - it's to keep you up to date on the band. This includes website updates, show reminders, and other random news like "hey, the new album is out".
You can unsubscribe at any time by coming back and entering your address in the same box, clicking the "unsubscribe" circle and hitting "Go". Basically, the same way you subscribed.