Canadian carrier Rogers has mentioned on its website that Android 5.1 for the Galaxy S6 and S6 edge will be released by June 2015, the website does state that this is an estimated release timeframe so the possibility of a change can not be ruled out yet.
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Although Google launched Guest Mode in Android 5.0, Samsung is yet to implement the feature in its latest devices. The Camera2 API - which enables CMOS camera sensors to shoot in RAW - was also not incorporated into Samsung's stock camera, with the Android 5.1 update slated to bring both features.
Android 5.1 isn't really a major update so users shouldn't expect any drastic changes, however they will find performance and stability improvements as well as crucial bug fixes.