Is there a published best-practice or recommendation for how often the user interface of a mobile app (professional/enterprise not consumer) should be totally redesigned? If so, could someone please provide a reference.
In terms of UX, you should redesign when necessary, and that should be continuously.
That's not to say you redesign the entire site continuously but rather you constantly redesign and improve the user experiences that make up the site.
Sometimes that might be tweaking content on a particular page. Or re-arranging the flow of a wizard based on user feedback. Or changing the button sizes on a screen after doing some user testing.
In other words, ideally, a UX team has the ability to make continuous improvements to the site as needed.
From a visual/branding standpoint, the question then becomes one of marketing and advertising more than UX. You don't typically want to redesign the entire site for the sake of having a new look as that's often an annoyance to the user. But there may very well be marketing reasons to do so. Sometimes marketing trumps. :)