My views on this subject have evolved during the 15+ years I've been flying sims. Back in the early days 15FPS felt wonderful on my 386 machines under MSDOS. As machines got faster and I got into the 30FPS range, I was of the opinion that there wasn't much difference between 15 FPS and 30. I still find anything above 15FPS adequate, but I need 20FPS or more for confident gunnery.
Nowadays, if I EVER see my framerate drop below 20 (even for a brief moment under the most complex and crowded conditions) then I immedately make some kind of adjustment. Usually this is just tweaking details downward a bit, but sometimes it means buying a new video card or faster processor.
My current machines are all running under LINUX, and that costs me about 10% in Framerates (because they are emulating DirectX). Nevertheless, with today's powerful hardware, my framerates range between about 50 and 500 FPS. Typically I cruise at about 100 FPS when flying level. The framerate dips down a bit when I lower my nose toward a crowded airfield, and it quickly goes up above 200 FPS when I am looking straight up. When I'm down in the weeds over complex, tree-filled terrain with several other planes in view it drops down to 50 or so. I never have to worry about dipping into the 20s anymore.
Regards,
-Peabody-