I can only say about my subjective experience and in non controlled situations.
Having said that, it is only in highly stressing situations that you will get close or over 16 GB.
This could be a combination of a payware detailed aircraft like the Fenix A320, in a 3rd party detailed airport or low flight over very detailed scenery and traffic addons.
With some freeware scenery that you find on flightsim.to consisting of multiple downloads with several GB each, you end up with a poorly optimized scenery, that FS will have difficulty to load in real time as you fly, and will eat away VRAM sometimes easily passing 16GB.
Typically it uses between 10 GB to 14 GB, even with the Fenix in most situations.