The standard (least from what I've seen on You Tube, is 30 FPS (or better). Some people are locked in at 30 FPS and have a blast flying! Others try to get every extra frame they can. From my perspective, FPS fluctuates like the stock market, it can go up and down like crazy, just depends on where exactly I am flying and what else is going on.
I've been flying in NYC and getting 50-60 FPS, but then I will fly over a crowded section of the city and have it drop to 31 FPS. I am still going very smooth and if I did not have the FPS counter going, I would not have noticed.
It honestly depends on what you are looking to get out of the sim. :)