with Apple offering batteries for $29, you should replace it every year!!!!!

Based on what I have heard I think that $29 will be a one time occurrence. The offer is only good till the end of the year anyway. IF you pay the $29 and that battery is found to have an issue or too high of a cycle count then they will replace again. But, if you just want to walk in and ask for a new battery and there is nothing wrong with your, they are not going to replace it for $29 again.

I just replaced one myself in my daughter iPhone6. In her case her WiFi was never working great and the GPS wasn't working either. Issue came up a few months after Apple care expired. I figured I had nothing to loose and bought parts and tool kit from iFixit for the antenna. I bought a battery as well since replacing the antenna removed like 95% of the internal parts anyway. The batteries are held in place by glue strips, so that was a bitch to get out even with the other parts removed. There are pull tabs on the battery to stretch the adhesive strips to break the bond, like command strips, but they broke, which is somewhat typical. So hair dryer, dental floss and plastic card. after doing it I would have also bought the tool they have that heats up in the microwave to loosen the glue holding the battery.

All is working now 100%. Now no rush to upgrade her phone for navigation when she gets her license soon.