How long is it supported? I’d hope for at least 5 years - as it is the price of a decent laptop.
Generally, iPhones are supported for five to six years. . . But work much longer. They are likely to need new batteries before then. . . After that, they will not accept an operating system upgrade because their hardware is not up to the demands of the new OS.
iOS 13, the version of the operating system that lands in a few days, works on phones back to the iPhone 6s, which was introduced in September 2015. That means it will be fully supported at least until the next version is released next fall. Most existing apps will likely continue to work for at least a year or two after that.