how long will iphone 6s plus be suported?

Even after it is ineligible for iOS updates, Apple continues hardware support for at least 5 years after manufacturing if the device is discontinued - Vintage and obsolete products - Apple Support

The iPhone 6s was completely discontinued in the fall of 2018, so should be eligible for hardware support at least up to fall 2023.

No way to really know, but the hardware is getting old. Apple tends to discontinue based on processor performance being inadequate for newer versions.