At what percentage of battery health do i have to replace my battery???

Which model? Right now with Apple promotion they will perform the replacement at any time for covered models, but it's supposedly just one chance without going under the standard 80%.

Traditionally, Apple would only perform the replacement service if the battery was under the standard industry standard, which was under 80% battery health compared to "design capacity". Or if the battery completely failed or is otherwise giving a message to service the battery.

If your phone's charge state turns off automatically at 20%, it's time to replace the battery and if your battery has swelling. In this case, you can repair from the nearest Apple Authorized Service.

I'll just add that when to replace the battery is up to you. A battery can have a short run time due to loss of capacity but otherwise still work properly.

Neither condition was reported by the original poster.