Every 3-4 years. My first two were iPhones (4s and 6). They used to belong to my brother, and I bought them from him when his contracts for them ended. The first smartphone I bought new (my 3rd and current one), is the Samsung Galaxy A50. I bought it because the battery in my iPhone 6 was fucked and there were new apps and games that it couldn't run. I don't miss having an iPhone.