I've seen image retention on LCD panels displaying a static image. This was on a computer where window outlines left a faint image. As the display was in the same static state for years and just put to sleep mode.
It takes a considerably longer period of time to develop than tech like plasma and CRT. I'd say when you step out on break or for lunch. Let the screen saver run rather than immediately putting it to sleep. Just setup a hot corner or keyboard shortcut.
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Samsung recommends not allowing a static element for more than two hours at a time. Although that seems overly cautious.