While Android 6.0 Marshmallow was just announced a little while ago, the actual OS has not been announced or made available just yet. Developers are still playing around with the third developer preview however. So we have no idea what devices from which manufacturers will be getting the update to Android 6.0 Marshmallow, however, we can likely say that all flagships in 2015 will get the update. According to SlashGear, these Samsung smartphones and tablets are slated to get the Android 6.0 Marshmallow update, the question is when.
"Galaxy Note 5, Galaxy S6 Edge+, Galaxy S6 Edge, Galaxy S6, Galaxy S6 Duos, Galaxy Note 4, Galaxy Note 4 Duos, Galaxy Note Edge, Galaxy Tab A, and Galaxy Alpha."
So that's basically all of the devices from this year and the end of 2014. Going back to the Galaxy Note 4 from last fall. It's interesting that the Galaxy Tab S2 isn't listed here, but I'm sure it'll be getting the update as well. Now this is not a confirmed list from Samsung, we likely won't get that until after Google has released the source code for Marshmallow in October or November. So remember to treat this as a leak, as that's what it is. Sammobile also points out that no Marshmallow firmware has been worked on just yet, so these updates are still quite a ways off.
Android Marshmallow isn't a huge update, compared to Lollipop last year. For the most part it is improving on what Lollipop did in Android 5.0. As well as fixing a number of bugs. Google announced Android M at Google I/O this past summer, and we should see the final release in either late September or early October. Alongside a new Nexus or two. Currently the rumor is that LG is making a 5.2-inch Nexus, with Huawei making the larger 5.7-inch model. Of course, those are rumors and we won't find out if they are true until Google announces them in late September or early October.
Samsung will need to get their hands on the source code for Marshmallow and then get it integrated with Touchwiz before they can push it out to their devices. Not to mention they'll need to test it and be sure there are no bugs, or very little bugs. So it'll still take a while. I'd be shocked to see any Marshmallow updates from Samsung coming out before the end of the year.