Currently, only a small percentage of Android devices are running a version of the latest release of the operating system called Android 6.0 Marshmallow. Those include Google's Nexus devices, obviously, but some OEMs like HTC, Motorola and LG have already released the update for some of their own devices. Samsung usually takes some time before the company starts updating its devices, in fact, there wasn't an official list of devices that were going to get updated. Then again, the company has been testing beta software in some regions, so we know all of the changes that the company is including in the update and alongside the changes included by Google, Samsung customized some visual aspects of the UI. A few lucky Galaxy S5 phones have even been updated to Marshmallow already, so we know that the update has to be getting closer to its official release.
There have been a few leaked documents with Samsung's plans for the Marshmallow update which displays which devices are planned to get the update and what the dates are for each device, although there's still no official confirmation on the leak. Now, another document has leaked, supposedly containing current information about the updating process for Samsung's devices. The document reveals that the update has been approved by the company so it should be released shortly, at least for unlocked devices. Carriers usually take longer before they release such updates because they need to test them on their own variants first. The first devices that will get updated, according to the document, will be the Galaxy Note 5 and the Galaxy S6 Edge+, and it could happen as soon as February. The Galaxy S6 and Galaxy S6 Edge could receive Marshmallow in February or March. The Galaxy S5, Galaxy Note 4 and Galaxy Note Edge could get updated in March or April. Once again, there's no way to confirm if the listed devices are getting updated in that time frame, but it doesn't sound very unlikely.
The past leaked documents showed that these devices would get the update in the first quarter of the year, as well as some more variants of the Galaxy S5 and some Dual-SIM versions of the Note 4 and the Galaxy S6. Still, there could be additional devices receiving the update in the following months. The new flagship devices, on the other hand, which are expected around March would most certainly launch running Marshmallow out of the box.