The “experts” will tell you to wait until a specific level.
But nobody over 25 actually does that.
I charge mine if I am not using it and it is below 50%, and I know that will allegedly wear my battery down, etc.
One thing I did notice is that the I-phone has a memory and if you charge overnight, it will slow charge so that it will finish right before you usually unplug it.
I’ve read random tech articles on the internet urging people to keep their smart phone batteries between approximately 40% and 80%. The idea is to avoid states of both low and very high charge.
However, when I got my wife’s iPhone battery replaced under warranty, the technician told us it is better to keep it as charged up as possible most of the time, and the bigger concern was running it on a low charge.
He also said battery preservation is a largely futile effort, and that these things are a perishable good with a finite service life. Although he favored keeping it charged up, he encouraged us not to go out of our way to change our charging habits.