When do I need to replace my iPhone's battery?

Replace the battery when the Battery Health drops to 80%. That might be a few weeks or a few months.....it all depends on how much you use the phone.

iPhone Battery Replacement - Official Apple SupportOfficial Apple Support › service › battery-power

Louie_Jay wrote:

but I'm not the person who is a heavy user, I'm just more on social media. I also don't use the cellphone often because I'm always workloaded

Regardless of what the battery health says or what the current capacity says, if the battery lasts long enough between charging opportunities to satisfy your needs or usage, then don’t bother replacing it. There truly is no magic number at which any battery must be replaced. If the capacity meets your needs or wants, just keep using it.

If it starts to give you issues, then consider replacing it.

Apple simply says the battery is “designed to retain up to 80% of its original capacity at 500 complete charge cycles.” What you decide to do at that point is up to you. ( Batteries - Service and Recycling - Apple ). If it reaches that point prematurely and is under warranty, Apple will evaluate if the battery was faulty (I.e. did not last the designed period of 500 cycles) and may replace it under warranty.

Apple recommends that you replace the battery when the Battery Health gets down to 80%. What you decide to do is up to you.

Apple says that if it less than 80% it may make your phone slower than its potential. So, you can wait for 2% more to get the best value.