I have used a cellular telephone from inception or the days of the Radio Shack, Black Brick phone. I also used a two-way in car radio telephone. I have personally experienced the technological changes for 30+ years, and what a revolutionary change it has been. I have always wanted the freedom from wireline technology. 

I've had my iPhone 13 Pro Max since it came out Sept 2021. Then it was stolen at a trade show in Vegas in April so I have an iPhone 13 ProMax (insurance replaced it with a Granite (gray) not Rose Gold that was stolen, to top that off it's refurbished). I'm waiting to see what they release this year, if it's a 14, I'm eligible for the trade in deal and that will mean I will have a new iPhone in a month :) 

15 years and before that about 17.  It works just fine, so why not keep using it?

My current phone is two years old. I use my phones until they don’t work anymore.

I used my phone until t-mobile gives a free phone with any working phone trade in. Still have a land line for business. 

Our company cell phones get updated every few years, but our landline phones it is much longer.  We went to voice over IP and it was awful.  Just going to totally digital phone systems for our main location.  We have had the same cell phone company for over 25 yrs, and get good deals on updates because we are on a very old plan.  The phones get rebuilt and resold - if they are still good enough technology. 

I've had my current phone for almost 2 years old.  Last phone was at least 6 years old.  I replaced it because it was running out of space and the battery wouldn't hold a charge longer than a day.  That seems to be my measuring stick these days.  If the battery won't hold a charge any longer, then it's probably time to replace it.

replace when necessary only

Land line or cell phone?  🤣😂😁 I have both…

Three-plus years. Generally, replace once every four to five years IF a newer version is compelling with new features. It's a TOOL, not a lifestyle.