It can be around 5 years, the iPhone 4S technically had the latest version of iOS from October 2011 to June 2016.
Also five years for the iPhone 5 (2012-2017). since iOS 11 supports the iPhone 5s, it will also get its fifth years of updates. (Hopefully more! Mine is still going strong, the arm64 processor was revolutionary.)
Five years does not seem to be an outlier for iPhones anymore.