When does a func() become too large?

I have a function that performs a lot of floating-point arithmetic. The function basically calculates coordinates on a fractal-type design of sorts. The function is called 3^n times where n is the number of iterations (consequently, the overall complexity of the fractal).

Now I understand the merits of modularizing things as a programmer, but my question is, at what point should the function be split into smaller functions, if the number of calculations within the function as it is will never change?