You should consider replacing your iPhone battery when its health drops below 80% of its original capacity. Apple recommends battery replacement at this point because the battery's ability to hold a charge and overall performance significantly decrease. Signs that indicate it's time to replace your battery include:

• Rapid battery drain requiring frequent charging.
• Unexpected shutdowns even when the battery indicates a charge.
• Slower device performance due to battery health affecting processing power.
• Physical signs like battery swelling or overheating.
• Frequent need for charging several times a day.

Apple's service or authorized providers can replace your battery professionally, ensuring quality and warranty. Alternatively, DIY kits and local repair shops are options but come with cautions about warranty and quality. In summary, check your battery health in Settings > Battery > Battery Health, and plan to replace the battery when the health is below 80% or if you experience significant battery-related issues affecting your iPhone’s performance or usability.