With something approaching 14 years of online experience, I'd say mid 30's for reasonable gameplay.
Mid 40's to 50's is smoother, and locked at monitor refresh rate 60's is awesome.
But its not all FPS, some of it is network connect, and some of it is having a clean lean running machine.
But speaking from personal experience, once fps are consistently below mid 30's I end up flying bombers, goon, or GVing a lot. Its simply not smooth enough to be enjoyable flying.