Depends on the environment and the people using them. We have 2616 and 2008 sets over 12 years old still working perfectly in an office environment. However I have seen phones destroyed in less than 3 hours after install by an "over zealous" employee. Also in manufacturing environments dirt and grease get in to it and gum up the works. Additionally, we have issues with women (I hope) wearing a lot of makeup. It gets on the handset, transfers to the hook switch and it starts sticking, same with buttons.
I also have 39XX phones that I think are 5-6 years old with the same issues as above.
I hope this helps.