In my experience, it uses about 8-12 Gb of my 16 Gb of RAM. I’ve read reports that those who have 32 Gb of RAM rarely see usage go above 14-15 Gb, and when they do it is usually going into big airports like NYC.
I was all set to upgrade my nVidia RTX 2060 to the new 3000 series coming out. I turned on the Developer tools in FS, and found that the vast majority of time I’m actually CPU limited. I’m running an i7 8700k, and FS is using all my cores. I have not (yet) overclocked my CPU at all, so I can probably squeeze a bit more of it if I want to.
I’m mostly flying airliners right now. When I’m in the “drone” exterior view, which is a free-movement camera mode, I get 50-60 fps in a wide variety of scenery conditions. But when I’m in the cockpit with glass displays I get closer to 30-40. I think a LOT of CPU cycles goes into calculating those displays. These numbers are all on custom video settings with a High-Ultra hybrid, 1440p.