The warranty period for iPhone battery replacements at Apple Stores depends on the type of service and coverage you have. Generally, Apple guarantees its service, including replacement parts, for 90 days or the remaining term of your Apple warranty or AppleCare plan, whichever is longer[1][3][5].
If you have AppleCare+, your iPhone battery is eligible for replacement at no additional cost if it retains less than 80% of its original capacity. This coverage continues as long as you maintain your AppleCare+ subscription[2][3].
For standard warranty coverage, the iPhone battery is only replaced if it is defective, such as swelling, within the first year of purchase. Normal wear and tear, like aging, is not covered under the standard warranty[2][6].
In cases where you pay for a battery replacement outside of warranty or AppleCare+, the replacement battery typically comes with a 90-day warranty[9].
Citations:
[1] https://support.apple.com/iphone/repair
[2] https://techbrowser.co/apple/iphone/iphone-battery-warranty/
[3] https://support.apple.com/iphone/repair/battery-replacement
[4] https://www.apple.com/legal/sales-support/terms/repair/generalservice/servicetermsen/
[5] https://support.apple.com/en-ph/iphone/repair/battery-replacement
[6] https://discussions.apple.com/thread/252875050
[7] https://support.apple.com/en-in/iphone/repair/battery-replacement
[8] https://www.apple.com/legal/warranty/products/non-ios-ipod-warranty-apac-english.html
[9] https://discussions.apple.com/thread/250377937