How do I find out how long a Samsung Android device will be security supported?
Unfortunately, like most Android phone manufacturers, Samsung does not give any guarantees about future security updates.

They do, however, have a web page telling you which models are currently scheduled for monthly or quarterly security updates:


Samsung Mobile Security - Android Security Updates


Judging from the current (2017-10) content of the list, it appears that the duration and timeliness of security updates depends on the price of the phone: The 2015 flagship model S6 still gets monthly updates, whereas cheaper models, even those released in 2017, only receive quarterly updates.