No no no...
None of you said the right answer.
@crispex,you are near to the right answer.
The human eyes can see 25 frame in each second and it means when your fps is upper than it then you have a suitable fps but if it goes upper then it means you have a good fps.
You can take a look at this page:http://en.wikipedia.org/wiki/Frame_rate
And I have toon boom animate pro(is a program for 2d animating)and in it's help it says the standard fps for 2d animation is 24 but as human can see 25 frame in each second then you can manually change the option to 25.
Have our guns was a mistake!!!
I am PaYaM