An internal memo at Apple has indicated that battery replacements for all eligible devices for the replacement program are now in stock and readily available. This means that if you are wanting to get your battery replaced in your iPhone, it should be something that can be done when you make a Genius appointment.
When Apple announced (or let it slip) that they were throttling iPhones with older batteries, the outrage was pretty quick and loud. In response, Apple began offering discounted battery replacements to customers who had batteries that were bumping into the throttling software inside of iOS 11. Normally that battery replacement is $79 but through the rest of this year, it is $29.
As you can guess, iPhone owners flocked to Apple to get their batteries replaced and in some cases, the batteries were having to be ordered. This resulted in multiple trips for the customer to their local Apple Store and weeks of delays.
Based on this memo obtained by MacRumors, those delays should be going away. That means that once you make your appointment to get your battery replaced, it should happen at that point and for most iPhones, takes about an hour.
The battery replacement program is for batteries that meet the criteria by Apple on the iPhone SE, iPhone 6, iPhone 6 Plus, iPhone 6s, iPhone 6s Plus, iPhone 7, iPhone 7 Plus, iPhone 8, iPhone 8 Plus, iPhone X that are out of warranty.