I'd say, as often as they want to. As you noted, friends have the option of filtering that content out if they don't want to see it.
I understand how it can be annoying; I have one friend who seems to post the almost exact same pictures a dozen times per day. But, it's their page, so I don't really have a right to judge. Plus, it's infinitely better than most of the crap that's posted on Facebook.