For the past couple of years, Apple have issued security and software (Safari) updates for two older versions only. So, right now, updates are issued for 10.10.x, 10.11.x, and 10.12.x. When High Sierra is released, I fully expect 10.10.x to drop off the update schedule. Hardware support for OS versions is another issue. Security updates are what drive my needs. When my hardware no longer supports a supported OS version, then it is time to upgrade the hardware, IMO.
So, to answer your question, I would expect Sierra (10.12.x) to be supported with security and software updates until 10.15.x (or whatever they call it then) is released, two years from now.