When I first got my S2, I got Image Retention easily after playing a game for 30 minutes. I started to get worried.
Well I took advice from everyone here. and ran slides on Vivid for 200 hours non-stop. After doing this IR HEAVILY improved.
If you don't play games a lot, break in is not needed at all. If however, you play games for hours at a time many times like me, break-in will very much help you.
At this point I have around 2150 hours on my S2. I can play a FPS...