Help! "Image persistence on Apple displays" issue

Never had this trouble with Macs before (Mac Pro, Cinema display), why now on my 8 month old iMac Retina 5K? Last night, I had used it for a few hours straight and noticed this issue afterwards. Also, how do I keep the screen saver going for long enough to fix it? I tried what Apple suggests, but eventually (after 30 mins, I think) the monitor shuts off...