Short answer: about 3 1/2 years.
Since the 3GS, I was replacing phones every year. Until the 7. I was a headphone jack holdout and hoped it would make a comeback and when it didn't, I got an 8. I skipped the X because it was 1st gen and I was hoping the notch would go away. When it didn't, I got a XS. Skipped 11 because I didn't like the bigger camera bump and hoped it would shrink. It didn't so I got a 12. I skipped the 13 because... superstitious... okay, just a little stitious... I got a 13 mini... I would still be using the 13 mini as a daily but I really liked AOD and ProMotion so got a 14. Skipped the 15 due to overheating. Skipped the 16 because the camera control button and phone kept getting bigger/heavier. Got a 17 Pro because it looks like they'll just keep getting bigger and heavier and foldier, and I like the cameras and I finally upgraded all my Apple accessories to USB-C versions, so phone was last to get an upgrade. So now I'm hoping as always perhaps a mini-esque phone is surprisingly announced, or if the iPhone Air survives, an updated version with additional camera for macro shots and stereo speakers. Otherwise, I'm holding out for some revolutionary 20th anniversary iPhone.