When should I change my iPhone battery and how much does Apple charge for replacement?

In Settings>Battery click "Battery Health and Charging" and it will tell you the maximum capacity. Apple recommends a new battery when this drops below 80%. The later OS's give a warning.

As for cost - not shure and depends where you live, but note that in Europe companies will soon have to allow 3rd parties to do this using Apple parts, so when it's time to change you should shop around.

Battery replacement for the iPhone XR costs $89.

iPhone Battery Repair & Replacement - Apple Support

There is nothing stopping you from going to a third-party repair place to have your battery replaced now. But, despite what was said above, avoid going cheap on this. Just pay Apple. There is no guarantee that anyone who is not Apple or an Authorized Apple Service Provider will be using genuine OEM parts. When it comes to things like batteries, cables, and adapters, going cheap can lead to problems down the road.