This is how long Apple supports iPhones for

Macworld
If you are thinking of buying an older generation or used/refurbished iPhone, or wondering whether it’s worth upgrading your current iPhone to a new one, one important factor to consider is how long Apple will continue to offer support for that handset. The good news is that Apple supports iPhones for a long time. In general, you can get seven to eight years of security-related software updates, while battery and other repairs are often available on five to seven-year-old handsets