The short answer is: Below 80%

As explained in this official Apple Support article, iPhone batteries are designed to retain 80% of their original capacity after about 500 charging cycles.

In fact, when the battery health of the iPhone goes below 80%, you'll see a notification.

To check the current health level of your iPhone's battery, visit Settings > Battery > Battery Health

Depending on the way you use your iPhone and whether or not you've activated "Optimized Battery Charging", you should not need to replace the battery before the first two years of its life.

And you, what’s the maximum capacity of the battery of your iPhone and for how long have you been using it?

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