Samsung hasn’t yet officially announced its Android 6.0 Marshmallow update schedule here is leaked RoadMap of Samsung Galaxy Android Marshmallow update schedule
Samsung Galaxy devices that will receive the Android Marshmallow update includes a number of Samsung devices and the time frame of the update. The Android Marshmallow update for Samsung Galaxy devices will be rolled out in two phases. In the first phase of update following devices will receive the update:
Galaxy Note 5 (SM-N920G) and Galaxy S6 Edge+ (SM-928G) are approved to receive Android Marshmallow in December 2015
Galaxy S6 (SM-G920I) and Galaxy S6 Edge (SM-G925I) are approved to receive Android Marshmallow in January 2016
Galaxy Note 4 (SM-910G) and Galaxy Note Edge (SM-915G) are approved to receive Android Marshmallow in February 2016
First phase also includes devices like Galaxy S5 (SM-G900G) and Galaxy Alpha (SM-G850Y) that are waiting approval for the Android marshmallow update and their is no estimated time mentioned in the leak for these two devices in first phase.
In the 2nd phase of Android Marshmallow update, following device will receive the update:
Galaxy A8 (SM-A8000),Galaxy A7 (SM-A700F),Galaxy A5 (SM-A500F),Galaxy A3 (SM-A300F)
Galaxy E7 (SM-E700H),Galaxy E5 (SM-E500H)
As there is no official details on yet.