At what battery percentage is it ideal to charge Iphone 13 Pro Max?

Charge your iPhone whenever you want to and for however long you want to. iPhone batteries, like all rechargeable batteries, are consumable components that become less effective as they age. There is nothing that you can do about this.

On the other hand, your iPhone 13 Pro Max has the capability to manage the battery's charge and discharge rate, optimally. This is done by the iPhone automatically.

My recommendation is to enjoy your iPhone the way you want to and let the iPhone worry about maintaining the battery. When the Battery Health reaches 80% or less (in about 2+ years), then get the battery replaced.

Here is some more reading on an iPhone battery and its performance ➔ iPhone Battery and Performance - Apple Support

Axel F.  

There is no such thing as an ideal percentage. Charge at any time, for any amount of time.

Do not repeatedly deplete the battery to low levels.

When to charge your iPhone or iPad - Apple Community