The technically correct answer to this question is 3 months. That is simply because a case being brought to an Employment Tribunal for unfair dismissal must be initiated within 3 months of the dismissal. So after 3 months, the company knows it is in the clear, in that employee has no potential claim on the employer.
Not good practice to be declaring redundancies then claiming business has picked up again. However I've known the technique to be used in cases of genuinely useless staff for whom the employer can't be arrissed to go down the disciplinary route.