The answer is: as high as possible without distorting.
Usually, this means a few inches below the collar.
Also measured as a few inches below the head of a tie, as done in various late-night talk shows.
This is a nice set-up, if you're cool with the lapel being on camera, because the lav mic is right under the person's mouth.
The vocals are perfectly placed over the microphone capturing the best possible sound from the lavelier.
Lavalier microphones are omni-directional, so the closer you can get it to the source of the sound (without distorting) the more clear & professional the recording.
In the image below, the lav could go up just a bit more - to get the best quality sound. Not necessarily too much. The placement of this lapel mic isn't wrong, but it could be improved as a matter of illustration. I'm sure the sound person will be okay if I use this image as an example, since I am that sound person 😛
Note where Bernie Sanders's mic is.