Rule of thumb:

12 fps for web animation

24 fps for quicktime: broadcast and film

Note for web animation: Some things will slow down the Frame rate such as tweening more than one thing at a time (tweening things on many layers), using alphas, many things animating at once, large panning backgrounds (particularly bitmaps), etc. Basically the Frame rate will be in part based on your CPU processor speed.

See the example below to see the actual Frame rate vs. the Frame rate set in flash. The Frame rate was set to 15 fps. (wait for one cycle to pass before seeing actual Frame rate)

ex.