How long should I use my Smartphone?

I got a iphone 6 which my mom owned for over 2 years. I cleaned it up, and am reqlly happy to have it. The performance is great and the 16gb memory is fairly enough for me. What do you think, I don't think that I'm gonna smash my phone soon. How long should I own it? (I'm 16)

That phone is good for another 6 years.

I usually keep a phone for 2 yrs or more and then get a new phone. But I can afford to do this.

Until it brakes and is impractical to fix, or you change service to phone company that you can't use your phone with, or your phone becomes very outdated like Galaxy s3 or iPhone 4 seem to be now.
If you do by a phone even if it is just because the new one is cooler, I would try to get at least 32gb? Might seems very optimistic but I think at least two more years

Really just as long as you want, after 4-5 years the hardware becomes so much better you will want to upgrade. Even if the battery eventually goes out just have Apple put in a new one and its like new again and its cheap to do so.

I used by iphone 3GS years ago for 3.5 years, when I finally sold it, it was still working great. I then used iphone 5S for 2 years and have 6S+ now because I really wanted the larger screen but I've had this 2 years now.

Note also when you sell an iphone you get decent money back for it also, I got $170 for my 5S back when I sold it, iphones hold their value well especially if you sell it within 3 years.