Android 6.0 Marshmallow is currently running on Nexus devices pretty much exclusively, as those get updated directly by Google. Other smartphone manufacturers take some time to update their devices as they modify the operating system to include some extra features and customize its looks. Some companies have already published lists of the devices that are getting this update, but almost none of them have mentioned when these devices are getting updated. The process of updating Android devices is more complicated than it may seem because there are many variants of the same model. Manufacturers need to test their software on some devices and after companies release the final builds of the software, carrier-locked versions of some phones need to wait for approval.
Motorola is famous for making soak tests of new Android versions on some devices in certain regions, they hardly change any visual elements from the software, so they are often among the first companies to update their devices. Sony has released a concept version of Android Marshmallow for a reduced amount of users, but it is far from being the final build, so they will be updating it with added features and possibly different aesthetics. Now there is news from Huawei about Marshmallow software. Xi Zhao, Huawei's marketing executive for their Honor division has stated that we should expect a beta version of Android Marshmallow in a matter of weeks for the Honor 7. The time-frame is rather ambiguous, but we could see this version in about a month.
Let's remember that Google partnered with Huawei to create the Nexus 6P, so perhaps this collaboration allowed Huawei to speed things up and customize the EMUI skin found on their devices a little faster. It is unknown if the beta build of the software would be released for additional devices or even the changes it will bring to the interface. Even if Marshmallow does not bring many visual changes to the stock interface, some companies could take the chance to improve some aspects of their skins. It is also unknown if the beta will be released to a select number of users or if they will have to sign-up to get the new software.