Considering that Google began rolling out the newest version of Android on September 29th, after a formal announcement, this really means that Google's own Nexus series will be getting them first. As always after an introduction to a new version of Android, the rollout can be slow to other types of smartphones and tablets, and this is what is known about the rollout of Android 6.0 (also known as Marshmallow or Android M) on Samsung devices like the Galaxy S6, S6 Edge, S6 Edge +, S5, Note 4, and Note 5.
The Independent states that back in July, Samsung posted something about how great Marshmallow is, but the company has yet to give any information about a release date even which phones will be receiving said update. Samsung has a pretty good track record of when it sends out its new updates, which can occur in weeks or sometimes months.
According to Tech Times, a new list has leaked which shows which smartphones will be receiving the update to Marshmallow, and it includes the Samsung Galaxy S6, S6 Edge, S6 Edge +, S5, Note 4, and Note 5, as well as the Galaxy A8 and A7. This leak emerged out of China, and some Samsung devices are on the edge and might not make the cut such as the Galaxy A5, A3, and the entire E series. Assuming the information is true, then the Galaxy S4, Note 3, as well as their older counterparts will not be getting the Android M upgrade.
At this point, it is not even clear whether or not Samsung will even release a formalized list saying which of their Android devices will be receiving the update. This is different from other companies, such as HTC and Motorola.
One can probably start a prediction based on last year's update to Android 5.0, also known as Android L or Lollipop. Although the rollout of Android Lollipop happened in the Fall of 2014, one of the newest Samsung Android smartphones, then the Galaxy S5, did not receive the actual update until Winter of 2015.
What makes this even worse is that of the major carriers (AT&T, US Cellular, Sprint, Verizon, and T-Mobile), only T-Mobile has been able to rollout the runner-up latest version of Android, version 5.1.1, to its customers.
Yes, this is a very slow rollout for the Galaxy S5 as far as new updates are concerned, which means that even those with the current and latest flagship phone from Samsung, the Galaxy S6, as well as its Edge variants, are probably not due for Marshmallow until Winter of 2015. The wait will probably be even longer for those who didn't purchase new Samsung devices this year. By that time the Marshmallow rollout will happen for Samsung devices for 2015, the Samsung Galaxy S7 will probably be announced at Mobile World Congress in 2016.