In the general case (I guess, you already know) the Nyquist criterion must be kept, also an anti-aliasing filter is necessary to avoid downmixing of above fs/2 input signal components.
The required sample length depends on the intended bandwidth. As with any FFT, windowing of the sequence is required to get a smooth filter shape. Simply, the window FFT is the filter transmission function, e.g. rectangular window (= no window function applied) has an unpleasant sin(x)/x shape.