At what percentage of battery health we can change the battery?

Apple recommends replacing the battery when the health reaches 80%.

iPhone Battery and Performance

https://support.apple.com/en-us/HT208387

If I remember correctly it needs to be less than 80% and within two years of purchase in order to be covered by AppleCare.

I have some older devices like an iPhone 8 plus that my elderly father is using and it’s below the 80% and it has a message displayed advising the battery should be replaced. We’ve decided not to do so since we’re probably going to upgrade the phones soon but it does definitely say that on the iPhone.