At what percentage battery health should I consider replacing it?

You should replace it when it causes you stress. If you are worried about the battery dieing before you can get back to a charger, then it is time. If you always have a charger near by then run it until the wheels fall off.

Replace it when your daily battery life starts becoming unacceptable or if the battery itself starts swelling.

Like everyone else said, replace it when it becomes a problem for you. Don't worry about the %life left, or how full you charge it. (IE some people try to keep it around 80% to extend the life of the battery)

Your phone is a tool, use it as such and replace parts when they become a problem. I'm still using a Pixel 3 which I'll upgrade later for security/updates.

6 hours still sounds fine. Wait a bit longer for it to deteriorate and replace. Probably, at something like 50 or 60% of battery health.