Threshold for phone battery replacement

In addition to Kappy's answer, it's when the battery cannot hold more than 80% of its original charge capacity.

• iPhone Battery and Performance - Apple Support
• iPhone Battery & Power Repair - Apple Support

For an iPhone in warranty, Apple considers premature failure to occur if the iPhone has less than 80% capacity at less than 500 cycles (1000 cycles for laptop batteries - Determine battery cycle count for Mac notebooks - Apple Support).

Officially, it's 80%. However, Apple will install a new battery if you insist upon it, even if your phone reports higher than 80%. My 6S was getting less than a day on standby, and just a couple hours with heavy use. Battery Health reported 88%. Despite this fact, the Genius Bar member did everything in her power to try to convince me NOT to buy a new battery. Finally, upon installing a new battery, I again get 2-3 days on a fresh charge with light use. Hmmm...Battery Health (beta) needs work.

1,000 cycles.

While still under warranty.