it depends on the kind of fractal you have.
your question is like: what is the cutoff frequency of a microstrip antenna!
and, by cutoff frequency, you mean resonant frequency?
I read some article relating the fundamental frequency to the perimeter of the main triangle of the sierpinski gasket fractal antenna. lambda (approx)= perimeter/2. The smaller self similar triangle will be based on you scale factor.
You can start off based on these dimensions and then run optimetrics to get closer to your desired frequencies.
Hey danesh how are you doing? Nice to see a guy from india working on it i am also working on fractal and its applications but not reaching to the end result. Reply me back so tht we can discuss?